Beyond Selidor

I should have explained myself better. When I talk about the human character 'becoming monstrous,' what I mean is that their outlook is changed, so they start to see the world less through human eyes than from the perspective of someone who has lived a long time and is unlikely to ever die (which, as I said in my previous comment, is likely to affect a person's perspective on history, mortality, morality, the passage of time and so on). I don't want them to actually become immortal or gain supernatural abilities (unless they had such abilities to begin with), I want their perspective to subtly change. Likewise with the immortal character — I don't want them to lose their immmortality, I just want them to gain a greater understanding of what it means to be human/mortal.

I listed a lot of my favourite examples above in my response to lirazel: Miryem and the Staryk lord from Spinning Silver, Vasya and Morozko from the Winternight trilogy (give me ALL the stories about young women, and the death gods who love them!), pretty much every mortal/immortal couple in Holly Black's fiction, and Nick and Mae from Sarah Rees Brennan's Demon's Lexicon trilogy. And while I don't think the relationships were happy or healthy for anyone involved, I still have a lot of affection for Buffy and her various vampire boyfriends. I also wish that Terminator: The Sarah Connor Chronicles hadn't been cancelled, because I think the show was going to go to interesting places in terms of the relationship between John and Cameron.

My favourite of all is Warden and Paige from The Bone Season series, though!