Roles in Each Book:
Jacob has a small role in the first book of the series. He is the son of Bella Swan's father's friend, Billy Black. When Bella uses him to get information on Edward Cullen and his family, Jacob tells her Quileute legends and introduces her to the idea that Edward is a vampire.
After months of depression over Edward's leaving, Bella finds friendship with Jacob and slowly begins to come out of her depression. Jacob is one of an ancient line of werewolves, a member of the Quileute tribe that has forever been the mortal enemy of vampires. Jacob undergoes his first transformation into a wolf in response to a new vampire threat, and he becomes busy patrolling the forest with his pack, searching for vampires in the area. When Bella is caught by the vampire Laurent alone in a clearing, she is saved by Jacob and the pack, who manage to destroy Laurent.
Bella impulsively jumps off a cliff and almost drowns, but Jacob saves her life. After Edward mistakenly believes Bella to have died, she and his sister, Alice, rush to Italy to stop Edward from killing himself, leaving Jacob hurt and angry. Jacob is disgusted by Edward's return and by Bella's willingness to take him back after he left her; he is also outraged when he learns of Bella's desire to become a vampire. Jacob reminds Edward of his tribe's treaty with the Cullen family, which states that the Cullens are not allowed to create new vampires.
Jacob attempts to distance himself from Bella by not returning her phone calls and refusing to see her, enraged by the fact that Bella plans to become a vampire. Later, Jacob visits Bella and Edward to discuss the vampire Victoria's return, and he tells Bella that he misses her and wishes that they can remain friends. With Edward's approval, Bella begins to visit Jacob on a regular basis. On one of these visits, Jacob tells Bella that he loves her and that he wants her to choose him over Edward. He forcefully kisses her, much to her displeasure, and she reacts by punching him in the jaw and breaking her hand.
Jacob's pack of werewolves and the Cullen family join forces to defeat Victoria and the army of vampires she has created. Before the battle, Jacob overhears Edward and Bella discussing their engagement, angering him greatly. Jacob threatens suicide and Bella, in an attempt to stop him, asks him to kiss her. Afterward, Bella comes to the realization that she is in love with him as well, though her love for Edward is much greater. The epilogue is written from Jacob's point of view; angry and hurt at Bella's decision to become a vampire, he runs away in his wolf form to escape his pain.
Jacob returns after an absence to attend Bella and Edward's wedding; though still visibly pained by her decisions, he tells Bella that he wants her to be happy. He is outraged when Bella inadvertently informs him that she and Edward plan to make love before she becomes a vampire, because he knows that Edward's strength could kill her. When Bella and Edward return from their honeymoon, Jacob learns that she is pregnant with her and Edward's half-vampire, half-human child and that she has become desperately weak. When he informs his wolf pack of Bella's pregnancy, their leader Sam plans to attack the Cullens and kill Bella and the baby because of the threat he believes the child presents. Jacob, who feels that the Cullens are innocent, disobeys Sam's command and separates himself from the pack, fulfilling his birthright as an Alpha wolf. He is joined in his new pack by Leah and Seth Clearwater, and they aid the Cullens in protecting Bella.
After the birth of Renesmee, Bella and Edward's daughter, Jacob finds his soul mate in her. He thus becomes extremely protective of Renesmee, whom he nicknames Nessie. As she grows, the two of them become very close, and Jacob acts as an older-brother figure to her. Jacob becomes a part of the Cullen family and the enmity between him, Edward, and the other Cullens is dispelled; he and Edward come to view each other as brothers instead of rivals