Ai didn't want to grow up. Or, rather, he didn't want to grow up in Hole. Instead, he ran away to pursue his dreams in another world - a world full of magic and danger. In this sense, he can be loosely paralleled with Peter Pan escaping to Neverland.
Officers Lost Boys
But he didn't go alone. On his way to the sorcerer world, he saved five homeless kids from abuse at the hands of sadistic bullies.

These were Dokuga, Tetsujo, Ushishimada, Saji, and Ton. They became not only the first members of the CrossEyes gang, but also its five core Officers.

Growing up on the streets exposed them to all the callous cruelty of the world.

Since they couldn't use magic, they were considered worthless, and society placed them on the level of humans from Hole. As such, they were nothing but test subjects for sorcerers, and could be mutilated and killed without consequences.

With nothing to lose, and their whole lives to repay him, they were Kai most loyal and zealous followers.
Yet they themselves couldn't explain why that was. We have no indication that Kai ever asked them to join him, ever forced them to do anything for him, or even proposed the idea of creating the CrossEyes gang. They did that on their own. Mesmerized by Kai's eerie charisma, they cheerfully allowed themselves to be pulled into a world of smoke and rotting flesh.

Perhaps it was because their childhood had taught them that life was cheap. Perhaps it was a desperate grasp at hope by an oppressed minority. Perhaps there was an element of revenge now that the tables have turned.

Whatever the reason, they retained an air of innocence about them, not unlike that of Peter Pan's Lost Boys. After all, what were the Lost Boys except a gang going around murdering pirates? Like the Lost Boys, the CrossEyes cast off the security of social norms to lead a more dangerous life of greater freedoms. But, instead of pirates, their rival "gang" was magic users.
It is doubtful that Kai cared about any of this. He was only interested in accumulating dead bodies. Eager to please him, his Officers killed and robbed for him. They welcomed new recruits and organized the gang's activities. They likely started the tradition of getting cross tattoos to show solidarity with Kai and to keep track of their members.

In return, Kai shared with the CrossEyes his expertise in medical science.

He taught them how to level the playing field against magic users through knowledge of their anatomy. By striking at points on their body which produce smoke, they could cut off their advantage in both abilities and range. In effect, this made the CrossEyes more efficient killers.

Kai also tried to teach them how to acquire the magical abilities of others, the same way he did, but all the attempts of his makeshift scientists to replicate this were spectacularly unsuccessful. Perhaps this was because Kai started out as a human from Hole, and the CrossEyes were sorcerers.
But Kai's farthest-reaching aid to magically-challenged sorcerers was the byproduct of his experiments: black powder.
Black Powder
Black powder came from the little devil-shaped tumors inside of sorcerers' heads. These devils were the source of their magic, and were probably also the anchors of their soul, since resurrection magic required their revival. Kai cut these tumors out of sorcerer heads to appropriate their magic - and he collected a lot of heads!

By taking black powder, sorcerers branded useless by society, or even those that couldn't emit smoke at all, had their powers temporarily amplified to those of pretty good magic users. This was another way to level the playing field since the powder did nothing for magic user elite. Maybe the amplification effects of the powder were limited by the strength of the magic user. Black powder was addictive, and respected magic users considered it shameful.

Interestingly enough, the Officers themselves never used black powder, though they ran an elaborate distribution chain for it.
Dealing Black Powder
There was an enormous market for black powder among the general public. It gave people a sense of self-worth, let them find steady work, and allowed them to protect themselves.

It also created new employment opportunities for drug dealers, and many joined the CrossEyes purely for financial gain.

The Officers weren't above using such individuals for their cause, but they did keep them to some basic quality standards. One guy who tried to sell fake powder was expressly concerned that he may be disciplined by them. 
we use them
Black powder was the main source of funding for the CrossEyes, but Kai only made a limited supply before he had to leave.
Despite their religious devotion to Kai, the amount of corpses at their disposal, and the care with which they must have tried to imitate his work, the creepy guys from Kai's lab weren't able to make more.

Everyone was getting desperate, but no amount of prayer and offerings to Kai's wax figure was able to bring him back.

To make matters worse, the lab was raided by En's Cleaners and burned to the ground.

When the last of the black powder ran out, things seemed bleak for the Officers.
In the years that their Boss was gone, the Officers did their best to keep the CrossEyes organization alive.

Dokuga acted as the unofficial leader, possibly because he was the only one who knew why Kai left in the first place and was the last person to have seen him.

Though they endured many hardships, never once did the Officers waver in their loyalty to their Boss, always clinging to the hope that one day he would return.

And one day he did.

Yet he didn't seek them out, and they were probably among the last to hear of it. On TV.

Granted, he probably had no idea where they were. But even when they contacted him by phone, overjoyed and excited as they were, he remained aloof and simply hung up.

Kai was a man of few words, so the Officers didn't take it personally. This only made them more determined to go see him.

It does, however, show Kai's tendency to take his Officers for granted. In fact, after the phone call, he immediately started operating on himself, fully expecting them to come up to his apartment and take care of him like mindless little robots.
Before the Boss left, he entrusted Dokuga with his favorite pair of daggers. It was an honor that meant the world to Dokuga, and one for which he was willing to risk the lives of his friends.

Yet when the Officers were finally reunited with Kai, he took the daggers back as if Dokuga was just a shelf he left them on all these years. He barely even looked at his Officers.
If Kai ever cared about his Officers, he certainly didn't act like it. And why should he? When it came down to it, sorcerers were nothing more than test subjects and body parts to him. He only saw them for the powers he could get from them, and the only reason the Officers and the rest of the CrossEyes were still alive was because their abilities didn't spark his interest.

After Kai decimated the En Family, the whole sorcerer world was turned on its head. Social outcasts were now in power, the magic user elite were on the run, and the masses flocked in droves to join the CrossEyes.
Powerful Presence
But Kai didn't do it for them. He only killed En because he wanted his powers, and because he was a potential threat to his goals. He only took over En's manor because it was a suitable base for the climax of his experiments. Any value he placed on the new influx of followers was that of convenient worker ants he could use to carry out his tasks. And they didn't mind, since they just wanted security and to get high off black powder.

But the Officers, in their sincere devotion to the Boss, did at times wonder if they meant anything to him. They all idolized him, and believed in the good he could do for their world, but they also feared him. Saji felt as though they didn't even exist to him, and that he only had eyes for some future goal that they didn't understand. Ton countered this by saying he believed the Boss cared about them since one time he sorta noticed him and shared a burger.

Nobody was troubled by the Boss' ways more than Dokuga. He consciously understood people's tendency to be "fascinated by a powerful presence whose nature they don't understand," yet he still couldn't help himself.
He was drawn to Kai like a moth to a flame. Even though he knew, better than anyone, what Kai was capable of.
Natsuki Risu
Kai only wanted power.

And he was willing to kill enemies and comrades alike to take it, without the slightest hesitation.
Kai's Eyes
But Kai wasn't some dumb sadistic brute. He was a hands-on scientist whose methods were as brutal as they were calculating.

To fulfill his ambition of becoming the ultimate magic user, Kai needed to amass more and more power, which meant he needed more and more bodies.

With nobody left to stop him, Kai launched the final phase of his preparations: wholesale genocide.
Even his Officers didn't expect this. Despite all their hatred for magic users, they never wanted to kill them all, and they privately expressed their doubts and disgust.

Nevertheless, they were on the frontlines of the assault, following their Boss' orders. They led the CrossEyes to mercilessly massacre hundreds of people a day to appease Kai, and directed an endless flow of body parts to be shipped to him from the field.

Many magic users fled and, to his credit, Kai let them. He instructed his followers to only go after those that stayed behind, since he took their confidence as indication of their abilities. He was mainly interested in the strong.

Kai was now amassing an enormous collection of heads. Mountains of them were piled up in his new and improved laboratory, their devil tumors cut out to feed black powder into a bioengineered suit.
No Knowledge
When Kai determined that he gathered enough ingredients, he initiated the Rainpocalypse. It never rained in the sorcerer world, but Kai made it pour, indifferent to the fact that it made the whole city writhe in agony.

His Officers managed to crawl over to his chambers in time to watch him be engulfed in a swirl of corpses and emerge as what I call "Darth Kai." It was a giant with four heads who breathed magic smoke and leaked concentrated black powder, yet was as incompetent as a newborn.

And so, the CrossEyes Officers were confronted with the harsh reality that the Boss they built their lives around, and were willing to kill and sacrifice themselves for, was a complete stranger to them, with unknown motives and questionable regard for their wellbeing.

Yet they had gone too far and done too much to give up now. Besides, he left them with little choice but to follow through with his mysterious plan.
By torchlight
It's unclear whether Kai expected to become an oversized baby, but what little coherency he had left prompted him to return to Hole, so it seems he didn't intend to stay in the sorcerer world.

He had effectively left his Officers to answer for his bloodbath after tearing down everything they thought they stood for.

Perhaps he also intended to take his Officers with him, but that is not the life they wanted. They truly believed that Kai could help create a less oppressive society. Now they were reviled as the oppressors, and even their own CrossEyes turned against them.

They weren't able to take a step without watching their backs.
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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.