Grimace was originally called the Evil Grimace.
The first version of Grimace was a villainous character with multiple arms who stole milkshakes. McDonald's later changed the character's design and personality, removing the threatening elements and presenting Grimace as a large, friendly, purple companion for Ronald McDonald and the other McDonaldland figures.
The change reflects a broader pattern in McDonald's branding: characters were repeatedly adjusted to suit family marketing and changing ideas about what appealed to children. The familiar modern Grimace is much less specific than the original character, which is why many people do not realize that he began as an antagonist.
Grimace's exact nature has often been left deliberately vague in advertising. That ambiguity has encouraged many playful descriptions, but the well-documented branding history concerns his transformation from the Evil Grimace into the friendly character known simply as Grimace.