I'm looking for a book I read either in the late 90s or early 2000s. The book was probably published around the same time, because I remember the illustrations on the cover looked "new" for that time. I know I got the book in my school library, so it was probably Young Adult.
I remember being obsessed with it back then, because the last book I read had a cliffhanger, and I never found any books continuing the story.
Here's what I remember:
There was a male and female character. Both in their teens or early 20s. The male was like an alien or he was human, but had been experimented on. The female was normal. The two had feelings for each other. It seems like the plot was based around the two trying to figure out what was happening to the male and why.
I remember the end of one of the books the male character is entering a "portal" and it seems like he was carrying someone in his arms. He smiles back at the female before entering the portal, and that's how it ended.
Seems like the book covers were greenish or had other neon colors incorporated, making it very sci-fi looking.