My favorite thing about Scion is the relationship between Joscelin and Imriel. First of all, seeing Joscelin through someone's eyes other than Phedre's makes him seem just that much more real. Anyone who's read the first three books know what Joscelin was willing to do for the sake of his vows and, later, for the sake of the woman he loves, and that there are myriad ways that he's worthy of love. Not to mention lust ;) But Imri's love for Joscelin comes from a different source, from Darsanga, from what he saw of Joscelin during their quest in Avatar for the Name of God.
On top of that, there's none of the ambiguity in Imri's relationship with Joscelin that's present in his relationship with Phedre. Imri's biological mother is still alive, and to say that she's a thorn in all of their sides is an understatement. Even as a teenager, however, Imri acknowledges that she is his mother, and he has to live with that fact. And he can't quite love Phedre only as a mother figure, because he's Kushiel's Scion and she's Kushiel's Chosen and there's always going to be that tension between them because of it. But Joscelin is untainted by all of that. Imri never knew his father, and so Joscelin is the only real father figure he's ever known, except for maybe the priest at the Sanctuary of Elua -- who lied to him about his identity and his past.
Imri admires Joscelin. Multiple times throughout the book he thinks fervently about how much he wants to be like Joscelin. And their love for each other is so pure and uncomplicated by the things that make his other relationships - with Phedre, Ysandre, Sidone, Melisande (obviously), and the people he meets during the course of Scion - so frustratingly complex. And that love is great and it is without a doubt my favorite part of the entire novel.