Che is trying to be normal. He has a list, even. Of things he wants to accomplish soon, after moving to New York with his family.
However, Item Number One is: Keep Rosa Safe.
Seems normal right? After all, Rosa's just his five-year-old sister. Completely normal. Except she's also a diagnosable psychopath. Rosa has a long trail of seemingly innocent acts of killing on her small hands, and it's only a matter of time before he kills again, maybe this time a person.
Che has to protect his sister from the world- but he may need to protect the world from his sister.