Short Version
Grimace was once a multi-armed monster stealing everyone else's drinks for himself. One day, Ronald came to him and gifted him a blueberry milkshake with extra whipped cream.
"Happy birthday, Grimace!" Ronald chirped.
Grimace took the milkshake and gulped it, and a strange transformation took place both inside and out. His extra limbs shrank into his body. And he felt weirdly loved and cherished.
"Gosh, thanks, Ronald!" Grimace said with a smile. But the clown was gone; in fact, he hadn't been there for his birthday for years now. "Wherever you are."
He still had his Blueberry Milkshake with extra whipping cream, just like his friend would make him.
Long Version
Grimace was once a multi-armed monster stealing everyone else's drinks for himself. One day, Ronald came to him and gifted him a blueberry milkshake with extra whipped cream.
"Happy birthday, Grimace!" Ronald chirped.
Grimace snarled, "What sort of trickery is this, clown? I am the embodiment of a glutton; I don't have a birthday."
Ronald chuckled, "Don't be silly, Grimace, you big lovable oaf! Of course, today is your birthday! I wouldn't be celebrating it otherwise. It's part of my magic power to know everyone's birthday! And I'm pretty sure it's this very one!"
He set the cup on the floor of the cave and said, "Have a Blueberry milkshake on me!"
Grimace took the milkshake and gulped it, and a strange transformation took place both inside and out. His extra limbs shrank into his body. And he felt weirdly loved and cherished.
He even shrank a bit.
Grimace felt a sweet, satisfied smile creep onto his face.
"Thank you, Ronald!"
That was years ago. The day Grimace was transformed from a monster to a friend. But now, without Ronald, what was the point?
No one else in McDonaldsLand even gave him a chance. Sure, he and Birdie got along well, and most folks stopped screaming in horror at him, but he always felt deep down that he wasn't quite fitting in. That he would be left alone to snap and turn into that horrible soda-stealing monster again.
Luckily, that didn't happen for two reasons: one, because McDonald's briefly introduced the Grimace Shake. Obviously, to celebrate his birthday company-wide. The second came as a surprise. The Fry Kids, now Teens, came to him to hang out and play with.
They were different, for one, they could talk. Introducing themselves. Sunny was the yellow one, an outgoing leader who could command a room if she wanted to. There was Pine, the green one, who was rather shy but very friendly once you got close to her, like Grimace did, apparently. Then there was Oswald, the orange, one who was rather sporty and athletic. Finally, there was Moon, the blue one, who was going through a bit of a goth phase. Very black and bruting, but under the leather was a kind soul. Grimace just kinda clicked with the group. He wasn't sure what he brought to the table as someone who was always a few steps behind everyone else's thought train, but he was accepted and loved regardless.
Sunny even gave him a nickname, "Grames."
(Pronounced like names)
Sunny gave him a birthday blueberry shake, "Thanks, Sunny, and thank you, Ronald, wherever you are."