Roger, Lyra's constant companion in Oxford, is a young kitchen boy who's kidnapped by the Gobblers in the opening chapters of the novel. The two are BFFs – the narrator tells us that Roger is more or less Lyra's "devoted slave" (3.99). Once he disappears, Lyra takes it upon herself to travel to the North to save him from the General Oblation Board. We really only see Roger a few times in the book, so in some ways his absence is more significant than his presence.
Lyra and the gyptians successfully rescue Roger from Bolvangar at the end of Part 2, but ultimately Roger will become a character of sacrifice and a symbol of Lyra's (accidental) betrayal.
Roger's innocence makes his death all the more tragic. When Lyra asks Roger if he'd like her to consult the alethiometer, for example, Roger says he doesn't wish to know what the future holds for him:
"Well, I dunno. There's things I'd rather not know. Seems to me everything I heard of since the Gobblers come to Oxford, everything's been bad. There en't been nothing good more than about five minutes ahead. Like I can see now, this bath's nice, and there's a nice warm towel there, about five minutes away. And once I'm dry, maybe I'll think of summing nice to eat, but no further ahead than that. And when I've eaten, maybe I'll look forward to a kip in a comfortable bed. But after that, I dunno, Lyra. There's been terrible things we seen, en't there? And more a coming, more'n likely. So I think I'd rather not know what's in the future. I'll stick to the present." (21.55)
Though Lyra's goal is to save Roger, she ends up getting him killed when she brings him along to see Lord Asriel. Little does Lyra know, Asriel's grand experiment requires a child. He cruelly severs Roger from his daemon in order to harness the energy to create bridge to another world. By unwittingly leading Roger to Lord Asriel, Lyra fulfills the prophecy that she would become a betrayer.