identity

When the main character of a story has lost their memory, the plot typically follows their search to discover what they've forgotten. Zen, however, is an exception to this rule: his sole goal in life is not to restore his memory. While he is well aware of the gap, he has no desire whatsoever to find out who he was.

Zen: I may know even less than you... because I don't even know my own past.
Rian: Your past? [...] What do you mean about your past?
Zen: I have no memory of the last 20 years.
Rian: What?! Nothing at all?
Zen: When I came to, my mind was blank and I was dying.
Rian: Zen... wouldn't you like to know your past?
Zen: I don't really care.

For Zen, knowing about his past honestly matters nothing to him. It doesn't matter that it would explain his skills or how he came to desire destruction above all else. Zen only cares about things that can help him get what he wants, and anything past that simply doesn't concern him.

Of course, Zen doesn't escape the trope completely, and decides that he needs to know who he was after the events in the Ista Special Prison. Even then, he doesn't seek the answers for the sake of knowing himself; he simply wants to destroy whoever controls him. As Hakka observes, it's the first time we see Zen have a human reaction, even if his reasons aren't what one would normally expect.

Zen: I'm not really interested in my past, but... being
controlled... even for a second... I won't stand for it.

The only fragment of his past Zen recalls is being shot through and falling. When Zen and Hakka go back to Zen's so-called home, his "big sis" tells them what she knows about his origins, something Zen had never asked about in his time with Zendo. He had simply taken the name they had given him, based on what little he had told them of his true name ("Ze"), and let the past lie forgotten. It strikes me as rather fitting for Zen to simply use the name they gave him. He cared nothing about his origins, even though they were lost to him, and was detached enough that his identity was meaningless. He burned off the mark on his wrist to stop him from being recognized by it, thus erasing the only connection to his past and starting fresh.

While Zen doesn't know at the time, it is the first time in his life his identity has ever been his own. As he eventually discovers, he was once Zero, the first of a black ops unit serving Galay during its way with Amata to do its dirty work. To the Galayan army, and to Zen himself, he was nothing more than a human weapon, needing no more than a number. The only person who saw past that was Gia.

Gia: (The government... started the wars and
may hope that all super soldiers die in them.
They're not marionettes... they're human.)

Gia came to Zen to tell him the truth and to try and save him, the way he hadn't saved the black ops soldiers. Despite his good intentions, however, Gia fails to recognize that Zen has formed his own identity. Gia refers to him solely as "Zero," and decided to try and end his struggle because it was the "right" thing to do. Zen, however, broke free of society's rules in choosing his identity, and rejects Gia's help. Only then does Gia accept that he is no longer Zero.

In fact, the only person who saw Zen for who he chose to become was Rian. While Gia and Hakka see him as Zero, the human weapon, and Kyrie sees him as the unstoppable criminal Zen, Rian simply sees him as a person with freedom. While Kyrie tries to end the fight between Gia and Zen, it's Rian who decides things, simply by acknowledging his identity and snapping him out of Hakka's hypnosis. She takes Kyrie's bullet and dies knowing she has found her own identity: a person free to make her own choices. Her last word echoes the last thing "Zero" spoke.

While Blank Slate has a heavy focus on the search for Zen's identity in the second volume, I don't consider it the driving force of the story. The search itself is driven by Zen's need to stop being controlled, and once he has the answers he forgoes any connection to others in order to continue on the path he began on. Zen acknowledges his past, but under the terms of the identity he's chosen. He lives, as he has ever since his rebirth, to continue down the path of destruction.