Basically, 2017M is one of The Devil's many attempts to gain ownership of Cuphead's soul. 2017M was originally a shape-shifting demon ordered to take the form of Mugman and sent to lure Cuphead to one of The Devil's many hideouts under the guise of his brother so The Devil could nab Cuphead's ghost juice. It worked at first, until the real Mugman showed up, and the brothers proceeded to wallop 2017M without much effort, putting an end to 2017M's subterfuge.
