favorite shounen

A story about a boy and his brother and how he's out to protect him no matter what even if he's a s class killer or psycho he'd just want to protect him. Sadly he turned out to be f class and is only a burden on his brother he's unable to handle that and when given a second chance to reverse everything and save his brothers life he does it. Now with this second chance he'll have to raise 50 s classes if he wants to protect his brother and potentially the world.