Boxer is a boxer-like Bakugan, sporting a boxing glove on each hand and attire similar to a professional boxer.
A Boxer appeared. 
One later appears being used by a group of thugs to coerce Dan Kouzo into giving up Dragonoid and Cyndeous. However, Dan uses Kelion to defeat the thugs and their Bakugan, taking back his Bakugan. 
Small Brawl Stories
Marco Chezanello used a Darkus and a Haos Boxer, named Black Boxer and White Boxer respectively, in a brawl against Shun Kazami and his Fade Ninja. Fade Ninja created several clones to confused the Boxers, leading to them eventually knocking each other out. 
Boxer does not appear in the physical game.