Grin's rough story:

Grin was captured by humans when he was either very or quite young. When they caught him, the leader of the humans carved a smile on the dragon's face and called him Grin. One of his minions, or maybe his son (idk, again I don't know a lot about him), felt sorry for Grin and stitched the wound. A chain was then put around his neck, and he was locked up. It's unknown what the humans exactly wanted from Grin but whenever he didn't obey them he would be hit with a whip, slashed with a knife, punched or stabbed. They kept him locked up for many years and they abused and neglected him. Only the person who had treated the "smile" on Grin's face didn't mistreat him and gave him extra food when the others weren't looking. Grin was slowley going insane. Years without sunlight had made his scales dull, and the constant pain inflicted by the humans made him get a very strong hate towards them. One day after being hit another time, Grin snapped and broke free.(tbc)

He killed every human he could find while he was trying to get out of the place where they kept him. On his way out though, a big guard dog bit his nose and didn't let go. Grin shook his head fiercely trying to get rid of the dog but with the dog he lost a big piece of sking on his nose. When he was almost at the exit he encountered the leader of the humans. The leader managed to slash Grin's left eye with a knife before he was killed. When Grin looked up from his prey he saw that the human who had helped him was looking at him. He hesitated for a short while, snarled and then ran away. He now walks around insane, scarred, with the broken chain around his neck and always an insane grin on his face. He has an enormous hate for humans and will kill every human he find.

Theories:

Theory 1. Grin is a son of Thorn. When I looked closer at Grin's new design, I noticed how similar his design was to Thorn's. Grin might be captured when he was really young, that's why Lisa can't remember him

Theory 2. Grin died either shortly before he escaped or just after he escaped because of his injuries and was turned into a demon by Aurora. Think about it, why do his eyes look the way they do? Why was he suddenly strong enough to break out even though he didn't get much food and he had lost a lot of blood? And why are his scars always bleeding? Aurora probably saw that Grin could be useful for her, and decided to turn him into demon.

I like his story, it sounds very creepy and I can definitely imagine it happening. It explains why Grin is so evil and hates humans so much.

I also like the idea of him being turned into a demon by Aurora. That theory makes the most sense. And it explains why he has those weird ghost-like eyes. O_O

General information about Grin:

Grin can't breathe fire. He was captured before he learned it from his parents. His teeth, claws, spikes and horns are extremely sharp though and Grin is very strong. Grin is quite small for a dragon and can run and attack incredibly fast. He isn't a very good at flying though, because his wings are torn and he hasn't flown for a very long time.
Grin is very aggressive and loves to kill. The thing he enjoys the most is to see the fear and pain of his victims when he kills them. He also loves the taste of blood. Grin has a terrifying laugh and often lets out chuckles while he talks. He also often talks to himself.

I think that's all I know. If you have any questions, feel free to ask

I don't really like the theories. But if I had to choose one, it'd be the first. It makes more sense. I don't really like the second one, because as I understand it, Grin just became insane because of the humans, plus I'm tired of the fact that Aurora has to have something to do with everything. And about the eyes, maybe he was just born that way? I mean, Dark Soul (or whatever his name was. I'm talking about that wolf) also has eyes like that, and he didn't got turned into a demon or anything. Meta Knight from Kirby has eyes like that too when he has the mask, when they're normally just normal Kirby eyes. And I've always seen the blood as a design thingy.

@PSTP The second theory doesn't say Aurora is the reason he is insane. The theory only explains his appearance and strength. He was already insane before she turned him into a demon.

Oh, I see. BTW, could you check my latest comment on the b-day gift?

I already replied

Ah.

so bloody. 0_0