While reading the first three books in the Twilight series, you might have been debating whether Bella is really Jacob's soulmate. Well, Breaking Dawn gives us the answer to that question: No.
In the beginning of the story, Jacob is in agony over Bella's marriage to Edward and her impending transformation into a vampire. His pain drives him to rage against Edward and his family. When Jacob hears of Bella's mysterious illness and her sudden return from her honeymoon, he's convinced that Edward has turned her into a vampire already. Acting against Sam's orders, he decides to attack the Cullens in a blind rage.
But when he sees Bella in suffering, he realizes that his love for her goes so deep that not even the shocking revelation of her pregnancy can drive him away from her. He even changes his mind about her transformation. He used to think that he'd want her dead rather than a vampire. Seeing her dying in front of him, though, he only wants for her to live, no matter in what form: "…that's what counted, right? She'd still know me better than anyone" (17.145). His change of mind also propels him to act as chief Ephraim Black's rightful heir in condoning Bella's transformation into a vampire.
A pivotal moment in Jacob's full acceptance of his painful, unbreakable bond with Bella occurs when Edward hears the baby's thoughts and no longer thinks it's a monster. At first Jacob feels betrayed by Edward and once again "all alone" in his pain. He decides to try to imprint on someone – anyone – just to get away from Bella and to stop the pain. After an unsuccessful girl-seeking trip to Seattle, he finally resigns himself to the reality that he can't run away from loving Bella and the suffering that comes with it. He accepts that he'll be tied to her forever.
However, when her heart stops after giving birth to her child, Jacob senses that the pull Bella had on him has shifted. Following the new direction leads him to Renesmee. Looking in the baby's eyes for the first time changes Jacob's life forever. His life purpose becomes Renesmee:
The gravity of the earth no longer tied me to the place where I stood. It was the baby girl in the blond vampire's arms that held me here now. Renesmee. (18.133-134)
In the end, Jacob is not only able to keep Bella as his best friend, but he finds true love with Bella's daughter Renesmee. Awfully convenient, right? How did you feel about the way the Edward-Bella-Jacob love triangle resolved itself? Perfect? Too good to be true? Kind of freaky?
After Jacob hears about Bella's early return from her honeymoon and becomes convinced she has been changed, his single-minded desire is to kill Edward. Despite Edward's continued expressions of friendship toward him in New Moon and Eclipse, Jacob still harbors suspicion and hatred toward Edward and his "bloodsucker" kind. And he firmly believes that it's wrong for Bella to choose a life with Edward.
But when he sees how tortured Edward is over getting Bella pregnant and endangering her life, Jacob's rage disappears. The longer he stays around the Cullens, the more they become people to him:
This was the problem with hanging out with vampires – you got used to them. They started messing up the way you saw the world. They started feeling like friends. (14.185)
He takes to Carlisle for his radiating goodness, and grudgingly accepts Esme's offer of clothes and food. He even admits that she reminds him of his mom. Although Bella mentions to Jacob during her pregnancy that he's part of the family, it's not until he imprints on Renesmee that he sees himself that way. Jacob's full acceptance into the family comes with Edward's words to him at the end of the novel, "…Jacob, my brother… my son" (37.224).
Stephenie Meyer established in Eclipse that Jacob was meant to be the Alpha of the werewolf pack, but chose to pass his position to Sam, because he didn't want that kind of responsibility yet. In an interview, Meyer explained that she had to find a way to force Jacob to step up, because he'd never make the decision by himself (source). That point arrives in Breaking Dawn when Sam orders the pack to fight against the Cullens and to get rid of Bella's baby, even if that means taking her life in the process. Forced to choose between his love for his pack and his love for Bella, Jacob chooses Bella and therefore rises to his calling as Alpha and splits from Sam's pack.
After the break-up, he considers himself a lone, homeless wolf, but not for long, because Seth and Leah ask to join his pack. At first, Jacob explains to them that there's no pack and that they should go home. He clearly doesn't see himself as a leader. When Jared meets Jacob as an ambassador from Sam's pack and entreats him to let Leah and Seth return, Jacob stresses that both of them decided to join him out of free will and renounces all claims of leadership.
Yet, Leah and Seth look up to him as a leader. Leah tells him, "I think you make a good Alpha. Not in the same way that Sam does, but in your own way" (16.52). His way of leadership seems to be unconventional in the sense that he doesn't believe in controlling his pack by exerting his authority as the Alpha. He trusts them to make their own choices. By the end of the story, Jacob's best friends, Embry and Quil, join his pack, and Jacob and Sam co-exist as Alphas of separate groups.
To find out more about Jacob, be sure to check out the other Twilight entries on Shmoop.