Leon is an assassin in a film of the same name. You may not've heard of it, because in
a certain country it got renamed
The Professional.
Understand, Leon is not a common hitman. He practices his work like a martial art, and leads a humble life. The only posession that he seems to carry with him, beyond the tools of his trade, is a small houseplant.
The film is about his relationship with a girl he looks after, when her family is killed in a Drug Related Incident.