M-O (Pronounced like the name "Moe", and meaning Microbe-Obliterator) is one of the two tritagonists in WALL-E, the other being Captain McCrea. M-O is voiced by Ben Burtt, who also voices the film's titular character, WALL-E.

M-O is a small, boxy, white robot with a retractable siren on his head and yellow eyes, and is equipped with a scanner and a rotating brush. He is stationed aboard the Axiom where he is tasked with cleaning, and he will do anything to keep things exempt of "Foreign Contaminants", including leaving the powered track he normally follows with the other cleaning robots. Once WALL-E boards the Axiom, M-O follows him around persistently, using his dirt trail as a guide, in order to try and clean him of "Foreign Contaminants". He eventually succeeds in doing so, after which, WALL-E befriends him.

During the events of the film's climax, M-O unintentionally wedges himself between the inner airlock doors in the garbage disposal area, allowing EVE to hold onto M-O's brush and save herself and WALL-E from being ejected into space. His tracking system also allows him to find the plant required to jump the Axiom back to Earth when the plant is thought to be lost.

Further reading:

Disney Wiki page