M-O (Micro Obliterator) is a robot that cleans the Axiom. He is very curt and efficent.

M-O cleans the Axiom

He chases WALL-E wherever he goes because WALL-E is covered in "FOREIGN CONTAMINANTS". Eventually, M-O joins WALL-E and EVE's "team", along with the malfunctioning robots, all the while making an effort to rid the Axiom of the "FOREIGN CONTAMINANTS" WALL-E holds.

It is possible M-O is based on Mo from the Three Stooges, due to his debut scene with the other two robots that still remain nameless.