It's hard to write about the relationship between Ai's several aspects without retelling the plot, but some context is necessary. He is a truly extraordinary creature, in body and mind. As a whole, he is both the protagonist and the antagonist of the story.
Ai Coleman was a quiet and withdrawn boy. He was raised by his grandfather, and was neglected by him. As far as we know, nothing particularly traumatic happened to Ai, and nothing particularly spectacular, either. His life was just... empty. Ai was a very intelligent kid, and the absence of stimulation in his life left him quite bored and depressed.

He spent his time reading books. Or at least one about sorcerers written by Dr. Kasukabe (Haze). He hated living in Hole, and he hated that sorcerers came there to experiment on humans, but he was also fascinated by them.

He might have felt that living
in their world and wielding
their exotic powers would
give his life some meaning.
And so, Ai decided he'd find a way to become a magic user.

Haze told him this was impossible, and brilliant a scientist though he
was, Ai was more clever. He did find a way. It required dunking his
human body into a lake of deadly toxic sludge, but he had nothing to
lose, and took the chance when he saw it.

Thinking Ai was about to die from his injuries anyway, Haze agreed
to perform the experimental surgery that Ai devised.
It saved his life, but he was never quite the same...
Cross eyes
After the operation, Ai developed a cross-shaped birthmark around his eyes. We don't really know why. However, the crosses are an important indicator of one of Ai's personality aspects: Kai. Ai having the crosses right away begs the questions: did the original Ai personality survive the surgery, or did the surgery fundamentally alter Ai's core personality to become Kai?

Haze said that "there were times" that he was scared of the recovering Ai. That could imply that there were times when he was not. And, indeed, we see post-operation Ai acting like he did before, even genuinely happy. Perhaps that Ai did survive. But he spent an awful lot of time staring at his reflection in the window, and when he turned around, the reflection didn't match the angle of his face. Maybe it's more likely this was the point when he internally developed the Kai aspect. And maybe the crosses weren't there all the time either, only when the Kai aspect reared its head.

Neither the "Splosh" Ai who stayed in the internal dimension nor the "melted-face" Ai who was found dead in the gutter after leaving the hospital had crosses around their eyes. But, even assuming Ai isn't identified with crosses, the link between the two is still very strong. The heads bearing crosses never tried to act against Ai's wishes. They had his medical skill and did his bidding. Ai's control over them was much more direct than over a head without crosses.
Kai smiling
Kai psychotic
Kai is the personality that dominates the heads with crosses around their eyes. He was Ai's badass alter ego.

If not being psychotic, Kai had his moments of lucidity when he spoke, but only when absolutely necessary, and his words were always brief and to the point. I haven't seen him show any emotion except murderous excitement, but he inherited Ai's methodical intellect, which made him very efficient at anything he did. And he really only did two things: eating and killing.

Kai was Ai's active hand in the world. He accumulated powers by hunting sorcerers, cutting the devils out of their head, and stuffing them into his own. He obsessively counted them, as if he has a quota in mind. Just as Ai's surgery threatened his sanity, Kai seemed to become more psychotic the more devils he added.
Kai normal
Despite his standoffish nature, Kai had a surprising amount of charisma and attracted a huge following among the common folk of the sorcerer world. They got tattoos matching Kai's crosses and called themselves the CrossEyes. Kai used the CrossEyes network to collect more and more sorcerer body parts for himself. The system worked like a well-oiled machine: CrossEyes would be sent to kill targeted magic users, then the heads would be sent to Kai's apartment and the bodies to his laboratory.

Kai himself started going after the elite members of the En Family. After he decimated most of them, En himself intervened, and Kai would've defeated him too if he didn't inexplicably start to switch heads in the middle of the fight. This cost him the kill, but he escaped unharmed. With En on high alert, Kai to switched to more secretive tactics that attracted less attention. Shortly thereafter students began disappearing from the Zagan School of Magic.
Carrying bodies
Aikawa no Mask
Enter Aikawa: one of the few heads without crosses. It's unclear when the Aikawa aspect was created. He could've existed before, he may have been the head who interfered in Kai's battle with En, but we only know of his activities from now on.

His defining characteristic was the coal mask that he wore in public. If he didn't, CrossEyes who have seen Kai's face would recognize him. And Aikawa himself was very aware of Kai's existence. He hated him, and was very hostile to the CrossEyes as a result. Because Aikawa was a wholly different creature from Kai. He was friendly, caring, and kind. He defended victims of bullying out of pure niceness. About the only thing he had in common with Kai was their mutual love of food.

Yet he was also one of Ai's pawns. He was Ai's passive hand in the world. The low-profile personality who could get close to sorcerers without raising suspicions and have them reveal things they normally wouldn't. In this way he was used to scout out potential magic users for Kai to kill. Aikawa caught onto this, but he couldn't help being an observer, and attending the Zagan School of Magic opened up a whole buffet of possible targets. 
Aikawa Mask
Risu and Aikawa
Given the sharp contrast between the Kai and Aikawa personalities, one has to wonder what they really are. Surely Ai can't randomly fabricate personalities out of nothing, so it's reasonable to think that they are aspects of his own. If Ai isolated the qualities that make up the Kai aspects while staring out that rainy window back at the hospital, would Aikawa be everything that's left over? Did he consist of all the qualities of Ai that could get in the way of his desires? If Kai was designed to be a remorseless killer, driven by an unadulterated lust for power that drew no line between friend and foe, then perhaps Aikawa contained all the human feelings and complexities that would question a fixed, single-minded goal.

Unlike Kai, Aikawa genuinely desired company. He had several acquaintances, but his best friend and partner was a fellow student named Risu. Since he couldn't emit any magic smoke, and going to school wasn't improving his ability, Risu assumed he was born without magic and joined the CrossEyes. Aikawa kept telling him to stay away from them, and called him a fool for believing that the CrossEyes' Boss wanted to help the magically-impaired, but Risu dismissed his warnings. Of course, he had no idea he was talking to Kai's own doppelganger, and Aikawa always took off whenever he felt a head-switching migraine.

Aikawa probably got so close to Risu because be believed Risu had no magic whatsoever and was therefore safe from Kai. He was wrong. One day Risu greeted him with a huge smile, confiding that he tested positive for the rare kind of Curse magic. Aikawa was horrified, but his expression was probably hidden by his mask, and he agonized privately how he had sealed his best friend's doom. What's worse, he knew it would be done with his own body's hands, by a man bearing his face.
Risu's severed head
Aikawa and Kai shared memories, and everything Aikawa saw or heard Kai did too. The CrossEyes sent Risu on a solo mission to a deserted park where he was jumped by Kai and bled to death from his jugular. But Kai misjudged Risu's magic. His Curse only activated after his death, and it immediately locked onto Kai to avenge Risu's murder. Kai tried evacuating to Hole, but Curse followed, and chased him through the back alleys.

All of Kai's skills and weapons were useless against Curse [who steals everything (tm)]. I guess I should add another emotion to Kai's personality: when it comes to Curse, Kai is terrified. Curse ripped out his sorcerer organs and tore off his head. Even headless, Kai still managed to paw around for bottles of smoke, and released Ebisu's reptile transformation magic.
Curse chasing Kai
This saved him from being devoured, but had two unexpected side-effects: it rendered him resistant to magic and erased his memories. The head that emerged in place of the one discarded by Curse had crosses, but was reptilian.

The magic resistance was the result of two layers of magic piled on top of each other: Ebisu's and Risu's.

The memory loss resulted from changes in Ebisu's magic due to the fact that she had been taking black powder for a very long time. In addition to reptile transformation, it now had the added effect of random memory loss or gain. When Noi was exposed to Ebisu's smoke, she lost all memory of her time as a reptile. When Ebisu used smoke on herself, she gained memory of a past experience. And when Kai cast her smoke on himself, he lost memory of his past.
Mirror Mirror
He was given the name Caiman (or Kai-man) by Nikaido. Just because he looked like that species, though he remarked how it sounded familiar.

Caiman was quite depressed at first, as Ai would be if he found himself back in Hole, but he was also very irritable, reminiscent of Kai's antisocial habits. Of course, both of these can be explained by his identity crisis from having lost his memories. He warmed up to Nikaido pretty quickly, though this probably had a lot to do with her cooking talents.

So who exactly was Caiman? He loved food, and especially gyoza, but this was an interest shared by both Aikawa and Kai. He was fun to be around and very protective of his friends, both Aikawa qualities. Yet mercilessly slaughtering sorcerers also came naturally to him and, unlike Aikawa, he did this out of routine, without passion or remorse.

He only felt pity once, and that was when a bagged sorcerer cut up by Shin begged to be put out of his misery. Sometimes the coldness of his attitude disturbed him, and he wondered if it was an effect of becoming a reptile.

But he had good reason to be angry at sorcerers. Sorcerers were hated in Hole since they used humans as test subjects, and Caiman took this personally since he thought he was one of their victims. He was searching for the sorcerer who enchanted him so that he could get his real face back. He knew that he had Curse living inside him, and he bit the heads of sorcerers so that Curse could determine if they were the culprit.

But there was no winning this game. Caiman butchered them whether Curse recognized them or not. And he hacked through countless numbers of them! So many that sorcerers started avoiding Hole. His magic resistance certainly came in handy. He did nothing with the bodies, though, and had he remembered his Kai self, he probably would've grieved over so many devil tumors going to waste.

Caiman's head was one normally ruled by the Kai personality. Despite the old Kai's head being torn off, it wasn't bound to a single one, and manifested in any head bearing crosses. He was filled with foreboding whenever he passed by the alley where he was chased by Curse, and he had nightmares and flashes of memory about how Kai stumbled upon Curse. He also had Aikawa's memories of meeting Risu at the school. Confusing, but not surprising, since the two shared memories.
Kai stabs Nikaido
The Kai personality also expressed itself in other, more direct ways. Caiman subconsciously retained Kai's medical expertise and intimate knowledge of sorcerer anatomy. For someone without prior training, he was frighteningly good at stitching wounds - a skill he honed as Kai, performing various surgeries on himself. There was even a time when Caiman blacked out and briefly switched directly to the Kai personality long enough to stab Nikaido. The wound was suspiciously nonlethal, and cut in a way that expelled the contract with En embedded deep in her body. Such a combination was too convenient to have happened by accident.

However, after visiting the Zagan School of Magic and getting more of Aikawa's memories back, Caiman started spouting blood and sludge from his orifices and actually switched from Kai's head to Aikawa's crossless head. It's interesting to note that, despite switching between such antagonistic heads, his personality as Caiman remained essentially constant. There was a slight shift: he became humbler, softer, and much less carefree, but this can also be explained by his remembrance of life as a sorcerer. This came as a huge shock since Caiman hated sorcerers and every medical test confirmed that he was anatomically human. He also began to feel as though there was something sick and horrible growing inside him, and could no longer even look at a knife without worrying that he might hurt his closest friends.

The smooth continuity of his personality between heads, and the mish-mash of Kai and Aikawa qualities and memories that he exhibited makes one wonder: what if Caiman was simply Ai in his most natural form? Whole Ai, incorporating the full spectrum of his personality, but free of his crappy childhood and magical ambitions. What if this is what Ai would've been like if he was exposed to warmth and meaningful friendships? What if he could've been happy in Hole?
Zagan head switch
Never Existed
Never Existed
Well, then there'd be no plot. And the plot must move on.

Ebisu's death dispelled the reptile magic on Caiman, Curse was liberated, and everything went back to the way it was before. Aikawa was Aikawa, Kai was Kai, and Ai was the creepy topless guy that lived inside their heads. All the past memories returned, and the memories of their life as Caiman were retained.

It was the great tragedy of Aikawa's existence that anyone he got close to dropped dead (though he was happy to see Risu alive again). He realized that Kai got wind of Nikaido's rare time manipulation magic through Caiman, so he acted like he didn't know her and avoided contact. It came off quite cold, but he did it for her own good, because he still very much cared about her. But he couldn't save everybody. He stumbled upon Natsuki, a young girl he befriended as Caiman, and learned that she had strong defense magic. Kai was quick to act on this knowledge.

Aikawa kept trying to do all he could to resist Kai and Ai, but it was no use. Ai told him he shouldn't struggle so hard since he never existed in the first place, but what did that even mean? Perhaps Ai meant that Aikawa never existed as his own person. Both him and Kai were created to be tools used by Ai to further his plans. But Aikawa was able to think independently of Ai. He was, in many ways, Ai's tortured conscience whom he severed to keep from being a hindrance.

He was forced to hurt more and more of his friends, and he despaired over his utter helplessness through it all. But he finally found a purpose for himself. If he couldn't resist Ai with his mind, he would strike a devastating blow to the whole by destroying his own body.
Never Existed
It was a cute idea, but it didn't work. Ai had the foresight to account for such foolishness. He watched Aikawa's drama with a cool sense of detachment. He sometimes played a pseudo-therapeutic role, checking in on Aikawa from time to time, making sure he was up to date on internal happenings, and even trying to teach him about his past. It's unclear whether he was simply preparing him for what is to come, managing his behavior, or if he actually wanted something from him. The only sounds Ai made were the "splosh" of gurgling blood, but Aikawa was able to understand him. And Ai was a lot more talkative than Kai.
Splosh Ai
This "Splosh" Ai was a ghostly presence who stayed inside the body's inner dimension. His appearance (when whole), was that of Ai frozen in the time when he was still fully human. He looked just like he did 15 years ago when he died in Hole's gutter, and was missing the top half of his torso to match the parts of his body exposed overnight to rain water. Therefore, he was likely the manifestation of that original Ai. He is the core self who puppeteers the others, and dispenses the various heads and body parts he has stored up in the dimension. He is calm, calculating, and scientifically-minded.
Splosh Ai
But he could also be a very frightening person, with a scowl that rivaled Kai's own. Though Ai was never exactly a ray of sunshine, he still had human sentiments and was relatively reasonable. This Ai was not. He had bouts of murderous intent that didn't spare even his old friend Haze. Haze, who didn't judge Ai's methods and, even now, sincerely offered to help him with his research. It seems that something very fundamental about the original Ai died - if not during the initial surgery, then after he was pronounced dead. The head of this reborn Ai was the face melted by rain water.

(The melted head was literally a zombie, and was just acting on its zombie impulses, but Splosh Ai looked pretty pissed too.)
Ai & Haze
Neither Splosh Ai nor Aikawa had crosses around their eyes, and they both were able to think for themselves. However, during Ai's transformation into a super being that transcended both humans and sorcerers, every one of his heads got crosses.

A fair question to ask is whether Kai was gradually gaining control over Ai all this time, and whether Ai's change was the result of Kai's "grim and sick" influence growing inside him ever since that reflection in the hospital window. After all, Ai was a dreamer, but Kai was ruthless ambition. Kai may have been a new force, born of sludge, and filled with an insatiable desire for power. Could "Kai" be the bizarre pulpy mass of crosseyes featured in the insert of Volume 10? It's hard to tell. Both Kai and Ai wanted this, but they all lost control in the end.
Crosses appearing
 Despite being marked with Kai's signature crosses, a new being was born: "Darth Kai." It could be significant that the only sound it made was Ai's own "splosh" noises. However, crosses and gurgling are where its similarities to either Kai or Ai end. The creature appears to be a blank slate, acting on instinct and showing basic emotions. It was designed to be perfect: four pairs of eyes to see in every direction, huge claws to defend itself with, smoke emission amplified by highly concentrated black powder... yet for now it was just a cumbersome baby.

But there are other players in this game. The Devils were quite curious about Ai's tinkering, and Chidaruma himself became excited for the first time in centuries. He saw Darth Kai's crosses and made a vague comment that they signified his seal of approval. Chidaruma also has a dark marking around his eyes - could the two be related? From what we know of the habits of Devils - their detachment from sentimentality and their sadistic sense of humor - Kai was acting a lot like a Devil. Indeed, the new being Ai created was meant to be an artificial Devil.
Chidaruma and crosses
It may be that the crosses which spontaneously appeared on Ai after his operation are a powerful and ancient symbol. In distant antiquity, the Devils made coins depicting the symbols of the three known worlds: Hell, Magic User World, and Hole. One of these coins bore a circle and cross very similar to Darth Kai's eye markings.
This very symbol was likewise painted on Hole's Central Department Store, which was built atop the pit of sludge that Ai dove into long ago. His infernal baptism in its mixture of pollution, water, smoke, and corpses filled him with the essence of Hole. It gave him the power of its corrosive rain and placed a replica of the whole city inside him. The crosses which appeared around his eyes may be the the outward manifestation of this absorption.

What is the significance of Hole's name? Could it be linked to the bubbling sludge pit - nay, the hole - featured so prominently in the insert of Volume 17? The same one in which Ai was reborn?

It transformed him into a sorcerer and, later, he returned to it to be transformed into a Devil.

Is this the primordial power of Hole carved into the Devil coins? The alchemy of science and magic. A sludge which induces mutation and drives evolution - the primordial ooze which gave birth to both Devils and sorcerers?

Well, given the ingredients list needed for Ai's rebirth, it is a "chicken or the egg" paradox at this point. But maybe there's something to it.
<<< There's Something Wrong With Boss HOME Kai's Lost Boys >>>
DISCLAIMER: all story, art, characters are copyright Q Hayashida. This is a fansite. No copyright infringement is intended. Translations by Gantz Waiting Room, VIZ Media, and sometimes myself. Dorohedoro is published in Ikki Magazine.