I don't think I have ever kept a character I have adopted for long, or at least as a major character in my group of them. Though, I was never much into adopting them myself, I was more of the one who sold them off. Though, technically, two of my characters were supposed to be adopts, but they were still created by me so I'm not sure if that counts. lol
Yeah, there's something more to when its 100℅ yours. Like, for me, I've been more likely to get attached to characters created from fandom phases than I ever have with adopts. I sorta use adopted character designs more towards background or minor minor characters.
Oh man, i forgot she was a Sgt frog character at first too! Love Marilyn even more. For me, all of my main five started as fan characters. Haru and Riku were from fucking Naruto, of all places, and Phil and Shiru were originally SGt Frog characters. And I'm not even gonna get started where Vion came from.