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From Unmariowiki

In his spare time, Mouser works as a Neo-Nazi, persecuting others and trying to create the Fourth Reich.

Mouser is mutated, legally blind rat with sunglasses and a bomb fetish who constantly harrasses Mario and his friends.

Originally, Mouser was the minion/slave of Wart, who had 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 King Koopa, who, along with making fun of Mouser, kept forcing him to wear weird costumes, under threat of the "duct tape". Mouser was eventually fired by King Koopa, who found Mouser's accent annoying. Mouser tried to get back under King Koopa's good side, but his plan to destroy holes in the ground was ruined by Princess Toadstool and a cheese log.

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 Little Mousers, Nomadimice, Squeeks and the Little Skull Mouser, only three of which are still alive, due to being massacred by Yoshi in a fit of rage.