I made this guy for a Changeling: The Lost game that never took off (and honestly im glad it didn't) so I decided to start retooling him for my more standard "medieval fantasy" setting. He was the son of a rich, yet reasonably well-adjusted family that was kidnapped and replaced by a Changeling on one fateful day. Used as a way of keeping track of his kidnapper's wealth, he eventually became something of a living treasure chest, and developed a very fine sense of mathematics. He performed dutifully, albeit only under threat of being made to perform worse tasks, and the thought of messing up beyond a certain margin of error causes him visible distress.
That being said, the moment he found a way to worm out from under his captor, he had more than enough sense to take it. He was able to refine the fae magic that bled into him in order to retake a mostly-humanoid form. The process wasn't perfect... but he could live with it. The one thing that had given him enough room to escape, was that he had never given them his full name, merely his surname. The fae owned the youngest of the Farnham family, but they could not lay full claim to Eustace in particular if he was no longer the youngest.
The experience left him changed in ways, some better, some worse. He had essentially become something of a changeling himself, able to take on different shapes, though the further they stray from either his "true" form or his human form he starts having trouble focusing.
Even if he had the mildly scrambled memories of his family, and knew he had been in that space, he realizes he can't go back to them, not with how long it's been, what he looks like now, and especially since... whatever it was that was taking his place, had been covering his absence. But he at least wants to be sure they're okay, and evade the capture of the fae that is now trying to look for either him... or the new-youngest Farnham of the family.