Good nickname for boy? Warning spoiler for book!?

Ok so I write books, and im planning on publishing one soon, but I need your guys opinion and help! Ok so I read the Percy Jackson series, and those are some of my fave books. WARNING SPOILER!!! DONT READ ANY FURTHER UNLESS YOU WANT TO KNOW! Ok so in the book Percy has a best friend, Annabeth Chase, and she calls him Seaweed Brain because he is the son of Poseidon. I have a character named Reuben. He trys to save people, is very into the shadows because he has to do a lot of things in secret, he leads people on hidden pathways, absolutely loves rivers/water and is very loving. He is courageous and brave, and will always stand up for friends in a battle. Now here is what I need help with, I need a name that his friend (who is a girl) will call him. I dont want anything cute like Roo roo I want something that she can call him to kind of be mean to him, but not like Doofus or anything. Just kind of like what Annabeth calls Percy in the Percy Jackson Series. Thanks so much!!!