Mouser in one of Bowser's weird costumes.
Mouser is mutated, legally blind rat with sunglasses and a bomb fetish who constantly harasses Mario and his friends. He could possibly be described as mentally unstable, at least following his time in prison.
Originally, Mouser was the minion/slave of Wart, who had him guard a warehouse and fight intruders, despite his blindness. Unfortunately, Mouser was pummeled all the time by intruders and was eventually fired by Wart, without pay and replaced by Clawgrip, a crab.
Mouser eventually got another job working for Bowser, who, along with making fun of Mouser, kept forcing him to wear weird costumes, under the threat of having the "duct tape" used on him. Mouser was eventually fired by Bowser, who found Mouser's accent annoying.
Mouser, after having exercised vigorously while in prison.
Mouser was eventually arrested, while being taped by the "Kingdom's Most Wanted", for years of dodging child support; it seems Mouser had had affairs with dozens of different people, impregnating them and creating the malformed species Nomadimice, Squeeks, and others. The bounty on his head was $89,000,000.
Mouser's time in prison hardened him and made him uncaring, driving him to attack others and exercise manicly. By the time Mouser was released from prison, he was overly-muscular and hateful of everything. However, before he could really do anything important, he died in the Battle of Tokyo in the Videogame War, due to being in the wrong place at the wrong time. He was only 98.
Despite this, he appears in The Faces of Volcanicity as one of the owners of the Volvic Revive Factories. It turns out that somehow his ghost made it back into his body.