Jacob is a Hybrid along side his sister Eve.
Jacob was born a year after Eve and a year after Underworld Awakening to both Selene and Michael. Jacob loves his sister Eve a lot and will do anything to protect her from harm. When he joined the military Eve became depressed and was worried about his safty but he comes home with a girlfriend who is a vampire and later marries her.
Selene: Jacob cares about his mother a lot.
Michael: His dad always punishes him if he screws up.
Eve: She is important to Jacob's life and is always protecting her.