Guys help me

I forgot the title of the manga that I used to read and it's a bloody/action type of manga. The boy is just a highschool student I think?? And she has a little sister (only a few years gap) which has gone through a medical operation and then when the boys birthday came the girl lost herself and became a killer, she almost killed her brother because she cut his finger and i think legs too?? And there came this group that kills this type of monsters and they take the boy with them to make him like them... Thats the least I can remember （/TДT)/ please help me