Panda

Note: He has real panda paw hands that reach his elbows, they aren't part of his jacket but hidden inside it where he takes out his claws. For an example of how they look, being with black fur instead, you can check out Atsushi Nakajima's tiger paws from the anime/manga Bungou Stray Dogs. When fighting they appear and reach his elbow like so: http://i8.mangareader.net/bungou-stray-dogs/8/bungou-stray-dogs-5491071.jpg

Panda, his nickname since his real name is unknown, is a fellow escapee from the famous jail that Aisuki broke him out of along with Zax. In exchange for freeing him, Panda pairs up with the bloody rabbit and causes mischief all over the town but also looks after him like a little brother, since said rabbit is still technically a child despite his twisted and insane mind. And to an extent shares his hidden apartment with Aisuki since the kid never had a home.

Panda has an unusual attachment towards Ai, much to his surprise, but he feels someone has to look after the kid and keep an eye on him if he's going way to far even if his an insane serial killer. Also out of sympathy since Ai did became a killer way to young and also the unknown reasons what caused the kid to loose his screws and the sudden breakdowns only Panda can manage to calm him down. But sometimes Aisuki is the one controlling Panda, in the sense that he can flipped Panda's switched to take out his inner insanity and killer ways and also turn it off if needed.

Panda became a killer when he was a teenager at highschool, when a gang tried to get his way with him but in exchange he killed them with a stop sign, this is when his insanity was born and soon taken to jail because of murder even if he was just defending himself to an extent and then took it to far. His teen years spended on a famous jail, the same one also tampered with him by some doctors, just like Zax and Aisuki, and gaining a few super and strange abilities and giant panda features on his body; like ears, tail and paw hands that reach his elbow.

Panda is also one of the few people that know Zax's real identity, discovering this after he hack through the jail prison files one day and finding Zax's information. The same files confirming Zax's false arrest of murder, actually being innocent but was arrested by the sole purpose that he was a very fitting guinea pig for the doctors and scientist on the lab of the jail next big experiment, being the recreation of life and a huge success with some minor defects on Zax's mind.