Leah's pain after her true love, Sam, imprints on her cousin Emily, has made Leah a bitter person. Becoming a werewolf in Sam's pack increases her suffering and bitterness, which only makes her unpopular within the group. When Jacob leaves the pack and her brother Seth join him, she seizes her chance to get away from Sam and asks Jacob to accept her into his pack. Their common pain over loving someone without being loved in return forges a close friendship between them. Jacob makes her second in command of his pack, which is something she's very pleased with.
Seth has become close friends with Edward after they fought Victoria together in New Moon. After Jacob leaves Sam's pack, Seth is the first to join him, because he sees the Cullens as friends and believes that it's his duty to protect them. When Bella fails to control her newborn vampire anger and attacks Jacob for imprinting on Renesmee, she wounds Seth as he's protecting Jacob. Edward describes him as "the kindest mind [he's] ever heard."
Quil is one of Jacob's best friends. After imprinting on Emily's baby niece Claire, his world revolves around her. He's concerned about Jacob's inability to get over Bella and encourages him to get a life away from her. He also suffers from Jacob's decision to leave Sam's pack, and eventually joins Jacob's pack.
Embry is one of Jacob's best friends. He isn't happy after Jacob leaves Sam's pack and eventually joins Jacob.