Maddox Selsam is the main antagonist in Double Down. He found Greg's balloon from the Balloon Brigade. He lives in the next town over from Greg, and he also plays the violin. He saw Greg's balloon dangling from a tree outside his window while practicing his violin. He lives in a greatly unpopulated area, where they have few neighbors as it is mentioned. When Maddox refused to let Greg touch his LEGOs, he gave him the separate leftover bin. Later, he notices a missing LEGO piece from the leftover set. He couldn't find it when Greg was playing with them, but it was later found to be stuck to his elbow. This led him to accuse Greg of stealing the piece, which resulted with Greg being kicked out of the house, ending their relationship.
Maddox has glasses, and wears a black shirt with white pants. He is also shown with what appears to be curly blonde hair.
Maddox has never played video games, and he goes wild when he sees video games. When he saw Greg play a video game for the first time, he gets too excited so his mother has to take him home.
He appears to be very careful with his LEGOs, since he won't let Greg play with them.
- Diary of a Wimpy Kid: Double Down (First and only appearance)
- Maddox could possibly be a representation of present-day children raised without technology and modern day comfort, since house has no television and has a large bookshelf instead.