Joining the Confederate Army in Texas before he was seventeen years old, Jasper Whitlock was the youngest major in Texas at the time. When he meets a mysterious woman named Maria, she turns him into a vampire. Set on creating an army of newborns (new vampires), Maria hopes to claim her place and dominate others. When Jasper was changed, he acquires the ability to feel and manipulate the feelings of others. Maria uses this by having Jasper control her newborn army. When she no longer needs them, she has Jasper kill them.
Eventually, Jasper grows tired of Maria and her violence. He doesn't want this kind of life; he lives with remorse for his victims and the newborns. When he discovers that vampires can coexist up North, he leaves Maria and becomes a drifter. Still unhappy with killing because he could feel his victims' fear and pain, Jasper tries to kill less often. His years with Maria had almost turned him into a savage, and his desire to feed was overwhelming.
So in 1948, when he meets Mary Alice Brandon, he finally feels hope again. Alice tells him she has been waiting for him, and how they can live off of "animal" blood in lieu of human. She explains that she has seen visions of a family called the Cullen's and how they will come to live with them. And with the help of Alice's visions, they find and join the Cullen Family in 1950. Shortly after, Jasper and Alice marry. Since Jasper and Rosalie, his "adoptive sister", share similar physical traits, they pose as twins; therefore, Jasper changes his last name to "Hale".
Although his "adoptive brother", Emmett, sometimes has trouble with the scent of human blood, Jasper is the member of the family who struggles with it the most. Not having the training or upbringing that his adoptive family has, Jasper has a hard time being around humans.