Well, I think Windkin got that way (in regard to his caustic personality) because the Forevergreen thing really hurt him, destroying some kind of innocence, although it's hard to understand that kind of innocence in a creature who is thousands of years old. But i've always had the feeling that it might have been the first time that he experienced misplaced trust, as well as people meaning him ill (that he can remember), and it would have been a terribly bad experience to have as the very first time such a thing happened to him, and that it just kind of did something to him, embittered him or whatever.
I also think that pretty much all the flying elves have this sort of dark quality, as though they're not always really happy like most of the others. I guess for some of them it may be due to the legacy of Winnowill (Aroree, Tyldak, and I really don't know about Aurek, who rather than being dark just seems to me, you know, very sophisticated); for Rayek, it's being Rayek, and for Windkin...well, he enjoys things the other elves don't enjoy, he goes places they don't go, he has experiences it's probably hard for them to understand when he shares with them. Kind of like Rayek, really. And I guess he pays a price.
I'd really love to know if that's him behind the sun symbol. It seems strange to me, though, that the same elf whose "immortal nature" wouldn't allow him to stay wrapped during the long sleep, wanted to be wrapped up and left for centuries, later on.