SPOILER WARNING!
When Jacob Black first appeared in Twilight (maybe I'm repeating myself, but anyway), I didn't think he'll pay such a major role in the series. He turns out to be a werewolf, an Alpha, a born leader, though he didn't want that birthright.
From the very begining, he's likable and easy-going and Bella often says that it's so easy to be with him. Thinking about him, I was struck by how much he changes since Twilight. Initially he is carefree and entertaining company, then after he learns that all the werewolf legends are true, he goes through a period of denial and fear of rejection. He becomes a bit more comfortable when Bella doesn't abandon him because he can now transform into a giant wolf, but his struggles have only just began. He's one of the characters (if not the one) that I empathise most with. He knows that he loves Bella and that she loves him back, but that she loves a vampire and is prepared to become one herself in order to always live with Edward. One of the problems is that vampires are mortal enemies to the werewolves and Jacob feels a real aversion for the person (ok, vampire) Bella cannot live without. In the course of the series, he manages to overcome that aversion so as not to lose Bella. He is devasted by the knowledge that she wants to become a vampire, but eventually, when her life is in danger, he would accept even that, so that she can continue to exist. Her personality does not change much after the transformation, so that is a comfort, but still, Jacob is ina pretty bad situation: his love is doomed: even though Bella loves him, she can never choose him over Edward. Jacob is more of a friend or family to her. He saved her from the desperation and depression following Edward's departure. Edward started tolerating him initially because of that.
But what can the future hold for Jacob after Bella's transformation? And then he imprints on Bella's half-human, half-vampire daughter and that drives her crazy.
I'll see how this one will turn out, but I think this is a pretty good outcome for Jacob, at least now he has any chance of being happy with a person closer to Bella than anyone. What's more, this imprinting bridges so many gaps: among the human, the vampire and the werewolf worlds. Bella thought the feud between werewolves and vampires unnecessary and Jacob's falling for a half-vampire girl emphasizes that idea. Maybe there will be a happily ever after for Jacob as well.