Mayhem is a dog-like creature with golden-yellow fur as a base color, blue fur on the top of its head, back and tip of its bushy tail (with the latter two being in a flame-like design). It has a jagged red fur-line on the top of its head, as well as the same red color around its red-brown eyes, resembling eyelashes.
Mayhem also has a red nose, a red fur patch around its neck, pointed ears (much like a cat), sharp claws, and sharp teeth with two fangs curving out, similar to a traditional Eastern dragon.
- April O'Neil: April rescued Mayhem from Baron Draxum, and he now lives with her as a pet. The two of them apparently have a deep bond.
- Raphael: Mayhem has a habit of scratching Raph's face repeatedly, despite the fact that Raph usually tries to be nice to him. In one instance, Raph saved Mayhem when the latter was trapped in April's bathroom mirror, and Raph's face was still clawed by Mayhem.
- Yōkai Council: Mayhem works for the Yōkai Council as an agent. Their relationship in depth is currently unknown but it seems that Mayhem is loyal to the Councilors.
- Baron Draxum: Mayhem was somehow in possession of a vial belonging to Draxum, who appeared to hold a grudge against Mayhem as a result.
- Foot Clan:
- Mayhem is named after show writer Russ Carney's pet cat.
- It is revealed in "End Game" that Mayhem is an agent working for the Councilors.