He was a failed experiment but he has a werewolf heart and with the help of Frankenstein he was able to get stronger along with when rai awakened him so I think he's becoming more and more like a actual werewolf
As far as I remember Frankenstein said something along the lines of M21 having Muzaka's daughter's (?) heart. He isn't related to Muzaka, its the owner of the heart that was transplanted to him as an experiment that is related to Muzaka (because it was her daughter's? not clearly specified at time time so I'm only sure that the person who owns the heart is definitely related to muzaka but not who it was exactly but there's a high posibility of it being Muzaka's daughter). Rather than failed experiment he is actually a successful experiment who needed a long time to work because the heart needs a lot of time to be in rhythm with its new heart which of course now allows M21 to use werewolves and resembles real werewolves...
"As far as I remember Frankenstein said something along the lines of M21 having Muzaka's daughter's (?) heart.".... He never did, i read this multiple times and it was never mentioned that he has Muzaka's daughters heart. For now it's been considered a guess by the fans because the author hasn't said anything on whose the real owner. Also Frankenstein and Rai only know that his daughter got killed. But nether of them no what exactly happen, what happen to her body, ETC. She still remains a mystery.....
Okay SERIOUSLY THOUGH...WHO THE HELL IS HIS M 21 GUY??!! HOW DID HE CREATE THAT WOLD SPIRIT WHILE ATTACKING THAT FEMALE WOLF????
IS HE SERIOUSLY RELATED TO MUZAKA OR SOMETHING??!!!!