The purple McDonald's character introduced in 1971 was Grimace.
Grimace became part of McDonald's expanding cast of advertising characters during the early 1970s. He was originally portrayed as a clumsy character with four arms who stole milkshakes, but the character was later simplified into the large, friendly purple figure more familiar to audiences. That redesign made Grimace easier to recognize and better suited to family-oriented advertising.
Grimace appeared alongside Ronald McDonald, the Hamburglar, Birdie, and Mayor McCheese in McDonaldland promotions. Although these characters were created for advertising rather than as restaurant employees, they became important elements of McDonald's visual identity. Grimace is sometimes mistakenly described as a monster or a milkshake, but McDonald's has generally treated him as a fictional character whose exact species is deliberately undefined.