hakka

At face value, Hakka is nothing more than a doctor in a small Amatan village with the unusual tendency to look after whatever injured criminals pass through. He is presented as a pacifist and a coward, unwilling to cause harm. He tries to hide Zen from the government, to little avail, and winds up becoming an outlaw with him, tied to Zen with the threat of a deadly virus.

Zen: Don't misunderstand. I never believed you about the virus.
Hakka: ...You knew...? But... then why...
Zen: I thought I might be able to see something interesting. [...] Someone righteous... falling into evil.

It's a simple reason to accept Hakka's presence and to go along with what he wishes, but it's one that falls very much in line with Zen's character. Their relationship initially parallels Russo's: he found Zen a compelling target, just as Zen is intrigued by Hakka.

Hakka, for his part, seems to have a different interest in Zen. While it's because of him Zen decides to search for his identity, Hakka seems to be looking for something else in him. His questions probe into the subject of Zen's humanity (or apparent lack thereof), never quite finding a solid answer.

As it turns out, Hakka's innocent act is simply to disguise his true identity as the very doctor who did the experiment that made Zen the unstoppable soldier he is. It's because of his hypnosis that Zero wiped out the black ops unit, and was shot (seemingly) dead, ultimately losing his memory. The act he put up was simply to use Zen for his revenge against Galay.

And that is the crux of Hakka's character: why does he keep up the act so well? The most obvious reason is to keep Zen from realizing who he was, but even then Hakka gave Zen all of the hints he needed to uncover his identity and lead him towards the result he wanted. At the same time, though, he says things that his disguise doesn't require at all, and seem to be more to himself than Zen.

Hakka: I feel... a little... safer. You're... not a monster. I've come to know about your past... and understand it. You have a history and connection to people.
Zen: I might have been a monster, you never know.

The question this presents is what's running through Hakka's mind as he asks these things. It sounds as if he's asking himself if, despite what the experiments to him, and despite the violence Zen has unleashed since his escape, there's a shred of humanity left in him. Hakka says in the last chapter that he never wanted to hurt anybody, and his conversations with Zen indicate that this is the truth. I feel like Hakka's questions are for a rather simple reason: to ease his conscience. He already knows he has complete control over Zen through hypnosis, but he wants to reassure himself that there was some good in his experiments all the same, and that Zen wasn't completely ruined. I feel that Hakka and Gia assign much more value to Zen's humanity than Zen himself does, but it's something Hakka had to decide on himself.

For his part, Zen is completely unaware of Hakka's history and feelings until the final chapter, but he does get to see Hakka's so-called descent into evil, even if he doesn't understand why it happens. When the truth is revealed, he reacts violently and is only stopped by Hakka's overpowering hypnosis and trapped in a battle with Gia. When he's released from it, though, he has the opportunity to kill Hakka and doesn't, citing a lack of bullets. Given the abilities Zen's displayed up to this point with his bare hands, it's clear this is an act of mercy. The reasoning behind it is left up to interpretation. I personally think that there's no reason for Zen to kill him since his control has been broken, and Zen is not a man of revenge, just one of destruction. He leaves Hakka's fate up to him, and the doctor decides to end his life himself. Whatever answers he was looking for no longer needed to be found.