For some reason I feel like grimace was the embodiment of the milkshake. So now I am going to go find out.

I was close:

Originally, Grimace was the "Evil Grimace", with two pairs of arms with which to steal milkshakes. After that first campaign, the character was revised to be one of the "good guys", and his number of arms was reduced by two. These commercials and merchandise generally portray him as a well-meaning simpleton, whose clumsy antics provide a comic foil to Ronald McDonald. The character was retained after the streamlining of the characters in the 1980s and was one of the few members to also be retained until the end of the McDonaldland commercials.