Ronald McDonald is the main character in McDonaldland. He has several friends (surprisingly) namely Grimace, Birdie, and Hamburglar.
Ronald was originally played by famed weatherman Willard Scott, who was later dropped because McDonald's did not want to have be represented by a fat person. Notice how thin today's Ronald is? It's no coincidence. In the original Ronald McDonald commercial, Willard said something to the effect of "I know you're not supposed to talk to strangers, kids, but it's ok to talk to me, because I'm Ronald the hamburger loving clown."