He was been brought in to No. 6 as a criminal locked up in the Lost Town. His criminal code is VC103221 and escaped the West Block facility.
Karan (Shion's mother) read the message about Nezumi's escape and his profile suggests that he was 12 yrs old, 145 cm tall, weighs 37.1 kg and has gray eye color.
Episode 4 reveals, his stage name is Eve and he is an actor.
Episode 8, the reason Nezumi doesn't like No. 6 is because the people before killed his parents.
He also sings very well but saying songs can only comfort people, not save people.
In episode 10, he showed his 'emotional' feelings.