Stiles' vessel is 12 when he realizes how stupid he's been. Humans grow. Stiles does not.
It doesn't take an incredible street smart demon(and yes that's exactly what Stiles is) to figure out that he's got a problem keeping up his little charade.
This is not really a thing he can plan around. He watches as the kids around him grow taller and older. He watches as the boys become gangly teens and the girls start growing breasts. Well, he doesn't watch in a creepy way, obviously, but it does help ones possibilities for observation to be able to teleport around town and hide in the shadows.
Stiles estimates that he has another year, if that, before he'll have to find a new vessel. The boy will die, but there is nothing Stiles can do about that now. He's been living on borrowed time anyway and Stiles has to look out for his own ass first. A boy who never grows older attracts too much attention.
But that winter everything changes.
Winter is the best time of year. Humans sweat less at winter, oh good grief, the sweating. Stiles will never get used to all these annoying bodily fluids. Of course winter has snot, but humans are generally polite enough to keep that to themselves. Stiles' body, of course, does none of these things, which is probably the only plus side there is to it because it's confining and restricting and - well scrawny.
But life on earth hurts a little less when you're protected by human skin; the sun doesn't burn as much and the multitudes of chemical and emotional impression that whirl around him is also dampened by the human's weak senses.
But oh yeah, that winter. Stiles had to adopt much of the boys personality when he took over the body. Usually demons didn't have to, because "fitting in" isn't really their main priority, but for Stiles it was necessary. There were memories and emotions too and Stiles wondered if all humans were this... Earnest? Passionate? Looking for word... Sappy. That's the one!
The boys memories clung to his parents, which was hardly a surprise. With all that human being was accused of being throughout time, sentimental was perhaps the most apt description. But there was also the girl. Lydia Martin.
She was plain, even as far as humans goes, red hair and pale skin. Average intelligence and no special courage to talk of. As far as Stiles could see the boy's obsession had started with the girl having a pretty smile. As noted above, sappy.
But that winter Stiles would come to understand just how powerful human emotions could be.
It had snowed. Crystallized water at it finest. Unless you count ice-sculptures, of course. Stiles once knew an erotic ice sculptor who carved this huge... well, it was grand and not very lifelike, one might add.
The frost had gone deep already, freezing lakes and streams solid and the kids of Beacon Hill were all outside. Apparently, it was frowned upon to engage in fistfights most of the year, but as soon as snow was involved it was perfectly legal to throw lumps of ice at other kids, roll them in the snow and then laugh. This fact alone might have doubled Stiles fondness for winter.
Lydia Martin was outside too, but she was dancing on the lake. Stiles felt a tug at his heart at the beauty of it and he knew it came from the boy inside.
With blades strapped to her feet she twirled and glided in elaborate patterns that was only in her own head. Stiles kept watching. Even after it grew darker and most of the kids went home he stayed to watch her preform. She was lost in her own head and the patterns became even more eerie and hypnotic.
And that was when the ice broke. Loudly and sudden, sending the girl into water so suffocating and cold that she couldn't even scream.
Stiles wasn't going to save her. There were still children there, watching and panicking as humans do, but they would notice if he was suddenly there out of nowhere, to pull her out of the black water. But then the boy inside reacted.
It was a demand so profoundly strong that Stiles almost lost control of the body for a fraction of a second, but it was frighting enough to send Stiles running for the hole in the lake where Lydia Martin had disappeared, sliding into the water as kids around him screamed.
The cold water had no effect on Stiles, but he knew humans were fragile and when he surfaced with the girl in tow she was lifeless. But the boy inside was relentless. Save her, save her now!
Stiles didn't have healing powers. A demon saving a human was outrageous, it couldn't be done. It had happened from time to time that demons had mended another demon with their essence, but humans were so different, all flesh and goo and sticky bits.
Do it, save her!
Well, Stiles thought, here goes nothing. Or even less than nothing, actually, because there was no way in hell it could possibly... oh shit, it was working!
Stiles could feel his essence splitting and it was painful and scary, but it was working. Cells, popped by the cold like rice krispies, morphed into a state that wasn't quite healed, but not quite broken either. It was the same emergency patch up that demons used on their own vessels to keep the body from rotting and bleeding out as long as they were still inside them, but Stiles knew it would only hold as long as some of his own essence stayed behind.
Do it! Keep her alive, the boy demanded and Stiles told him to shut up. This was walking on thin ice, pun intended, because it would mean that the Martin girl would be different. Not quite human, but not quite anything else either. She'd be stronger, smarter and better, but not so much that anyone would be able to call her a freak for it.
And we'll be weaker, Stiles told the boy inside. We won't be as powerful. We'll be open to attacks, be killable. We'll get sick and we'll - oh, we'llage.
It's worth it, the boy said and Stiles should have gotten out right then and there, because nothing in the world was worth dying for. Nothing. But Stiles couldn't get out. He was already in too deep, caught in the dad's brave attempts at keeping face in front of his kid while he was drowning inside in guilt and sorrow. Caught by the boy inside, his kindness and willingness to sacrifice himself for these random and brief creatures around him.
Stiles knew that if they could make it through alive and keep the Martin girl alive too, then Stiles could simply take his essence back later and be whole again, be immortal again. All he had to do was keep them alive.
And Stiles would age, the charade would be complete and he would be able to stay.
I'll do it, he told the boy and with those words everything changed.