After Bella married the vampire Edward, she became pregnant with their daughter. Sam, after finding out about this, decided to lead an unprovoked attack to kill Bella and the baby. Jacob and Seth, however, refused to follow Sam on this, and broke away from the pack to aid the Cullens. Leah, despite her dislike of vampires, decides to join them so that she can protect her brother and get away from Sam. Her choice actually hurts Sam, who sends a messenger to try to persuade her to come back to his pack. She wants to remain in Jacob's pack until she can stop being a werewolf, and despite the initial antagonism between the two of them, they soon form a closer bond due to their shared broken hearts. Her and Jaocb have a deep conversation one day during hunting, mentioning how being frozen in time has stopped her mensturation cycle and has disabled her ability to get pregnant. Jacob also remembers her breakdown when she first became a wolf, wondering what kind of freak she was, the "girly wolf". Leah also wondered if she maybe wasn't as female as she thought she was, and wondered how imprinting would apply to her. It is in this discussion with Jacob that Leah mentions how she can relate to Rosalie.
She is so loyal to Jacob that she goes so far as to confront Bella about her tormenting Jacob. Bella is very hurt by this and Jacob tells Leah to lay off her, but does not enforce it in his "Alpha-voice" like Edward would like him to. She does not enjoy spending time around the vampires, but does so out of loyalty to her new pack. After Leah and Seth join their pack, Jacob names Leah as his Beta. Leah, along with the rest of the werewolves, stand with the Cullens when the Volturi, an Italian vampire clan that are like royalty, arrive to kill Renesmee, Bella's daughter. Their efforts succeed in repelling the Volturi without a fight breaking out. The recent events have made Leah a much happier person, and she is now "pretty satisfied with life."Originally, Jacob and Leah didn't like each other. When Jacob and Seth broke off from Sam's pack and joined the Cullens, Leah decided to join them. Though they were originally hostile towards each other, Jacob and Leah gained a new understanding each other and became good friends. Leah was later named Jacob's "beta".
While some fans believe that Leah fell in love with or imprinted on Jacob during Breaking Dawn, Stephenie Meyer has said this isn't true. This would simply have made their interactions too complicated, since she started out disliking Jacob, then they gain a "friendly" understanding. If Leah had imprinted on Jacob at any time, it would have been the first time she saw him after her first transformation, and she would have had a compulsive need to be with/near him from that moment on. This is not the case. This also would have not been possible because Jacob imprints on Renesmee.