American McGee's Grimm/WMG

Grimm is Rumpelstiltskin.

 * American McGee has hinted on his blog at least twice that Grimm will be "getting his own happy ending" in the first game's Grand Final}. In the Little Red Riding Hood episode, Grimm references two of the original collectors of the tale, The Brothers Grimm, hinting that he himself is not the author of these stories, and that the name "Grimm" may be a pseudonym. Rumpelstiltskin is most often described as a dwarf of shadowy, possibly demonic origin, this particular version even makes mention of Grimm's trademark "little red cap." Furthermore, though Grimm holds many fairy tale characters in withering contempt, and isn't shy about demonstrating the extent of his wrath towards them, he often makes exceptions for heroines who are forced into dire situations through circumstances beyond their control. Rumpelstiltskin behaves in a similar manner towards the Miller's daughter, taking pity on her and bailing her ass out multiple times, even going so far as to alter the original terms of his child snatching deal to give the Miller's Daughter (now a Queen) a (slightly) better chance at success. It's been theorized that Rumpelstiltskin wanted to either kill the Queen's child, or make the Queen's child more like him, either or both of which are pretty much Grimm's modus operandi. And, in the original, unbowdlerised orally-told version of the tale, the defeated Rumpelstiltskin escapes to spread chaos another day, which would certainly qualify as Grimm's "happy ending." Grimm also has a tendency to sing, dance and rhyme on a dime when he is especially happy...
 * Alternate/Related theory: Hell Hath No Fury Like A Trickster Scorned. In one version of the tale, the terms of the deal Rumpelstiltskin makes are that he will teach the Miller's Daughter to spin golden thread, but in exchange, she must marry him. Grimm hates it when people welch on a deal, and may not have appreciated someone breaking his blackened little heart, so when the Miller's Daughter guessed his real name, he decided to make the entire fairy tale world a living hell as payback for the Queen not holding up her end of the bargain, and ruining his chance at happiness. Guess we know why Grimm hates weddings...