The cleaning robot that follows WALL-E on the Axiom is M-O.
M-O is a compact cleaning robot whose name stands for Microbe-Obliterator. He is programmed to remove foreign contaminants from the Axiom, so WALL-E immediately becomes an irresistible target after bringing dirt and Earth debris aboard. M-O races along the ship on a small roller ball, scrubbing the tread marks WALL-E leaves behind.
At first, M-O’s behavior is comic and single-minded: he is more interested in cleaning WALL-E’s tracks than in understanding the larger adventure. His pursuit nevertheless carries him through the ship and into the central conflict involving WALL-E, EVE, the plant, and AUTO.
M-O is not another version of WALL-E, nor is his name R-O, P-O, or N-O. The hyphen is part of the character’s stylized name, and the abbreviation reflects his sanitation function. He eventually acts beyond his original cleaning routine, helping WALL-E and EVE during the escape and return-to-Earth sequence.