Maddox Selsam is a character who appeared 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. He has never played video games, and he goes wild when he sees video games, as Greg played a video game at his house which was Maddox's first ever video game he'd seen. He freaks out so much that Mrs. Selsam has to take him home. His home doesn't have a TV and in the living room there is a bookshelf in place of a TV. His room contains tons of Legos that collectively are most likely expensive in value which his mom buys for him whenever he wants a Lego set. That means his mom may be rich. Maddox appears to be very careful with his Legos, since he won't let Greg play with them, and he also notices a missing stud lego brick from his leftover lego pieces bin. The piece goes missing when Greg was allowed to play with only the legos in the leftovers bin, and when he finished playing with them, the missing piece got stuck to his elbow. He accused him of stealing his Lego bricks, leading Greg to be kicked out of his house and 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.
- Diary of a Wimpy Kid: Double Down (First and only appearance)