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Keisuke has an extremely strong sense of justice. He is completely selfless in standing up for the unfortunate party. When "Hero" meets Keisuke during the Tokyo Lockdown, Atsuro comments that Keisuke has changed since middle school and has become rather timid.
After Midori gets her COMP and goes on to exact "magical punishment", Keisuke becomes extremely worried for her and tries to convince her to see some sense. Unfortunately, Midori doesn't hear him, and Keisuke learns that Midori will be attacked by humans whom she is trying to save. This made Keisuke very angry and he summons the Judge of Hell, Yama through sheer force of will.
"Hero" then saves Midori and has the option stop Yama's rampage and change Keisuke's mind.
During Yama's rampage, he killed one of the Shibuya Daemons. Kaido then goes on a hunt for Keisuke, though he didn't actually know that Yama was summoned by Keisuke. Based on player's choice, Kaido can be asked to help Mari defeat Kudlak, thus allowing "Hero" and his group to confront Yama and Keisuke.
Yama would be defeated, and Keisuke can be convinced that his method of upholding justice is too extreme. Thus Keisuke can be recruited to the group.
Keisuke later reveals the reason behind his personality change. During high school, he protected a particular student from bullies. But later, that same student bullied Keisuke in return.