Your younger brother's name is Tom.He's only 9 years old,he's obedient,mean,and arogant.He always wants to be by your side because you are the big brother,he wanted to do that but you always refused because of your own works.During an outing,he picked up a fang in abandoned house. Knowing it had magic,he looked it up in a book and it was a devil's tooth.He inserted it into his brain by performing surgery on himself.And it merged with him,turning him into a demon.He won't kill you.