Grimace is the purple, large, rounded McDonaldland character.
McDonald's introduced Grimace as part of its fictional advertising universe. The character's early presentation was more mischievous and was associated with stealing milkshakes, but later campaigns softened him into a friendly, childlike companion of Ronald McDonald.
That change in personality is a common source of confusion. Older commercials may portray Grimace differently from the gentle, word-sparing character remembered from later decades. His visual design also became simpler and more rounded over time.
Grimace is not the name of a McDonald's menu item or a description of the restaurant's purple branding. It is a proper character name. The character's unusual color and silhouette helped make him immediately identifiable alongside Ronald McDonald and the other McDonaldland figures.