Kaiman is the lead character of the manga. He appears to be the victim of a magic user, who laid a curse on his head.
Other than that, he has no recollection of his previous life. He has the head of a large reptile, with no other alternates in limbs or body. There are cross-shaped marks over each of his eyes.
It appears that despite having his head chopped off, he can regenerate it in a matter of time.
Kaiman does eventually get his human face back, along with his lost memories, but the truth about his identity was more than he bargained for. It turns out he was originally a human boy from Hole named Ai Coleman. Ai put himself through surgeries to become an artificial magic user, and his self has since fragmented into several quasi-independent personalities attached to multiple heads, including Aikawa and Kai. Kaiman actually wore one of Kai’s heads for most of his time in Hole, as indicated by the crosses he had around his eyes. He switched to Aikawa’s crossless head when he experienced a flashback at the Zagan school of magic. Kaiman was still wearing Aikawa’s head during his fight with En when Ebisu’s reptilian magic was dispelled, which is why he reverted to the Aikawa personality and not Kai at that time.