"Oh and one last thing never turn back on your enemy "
Jasper was turned into a vampire in 1863 by a vampire named Maria when he was 20 years old. As a vampire, he gained the ability to sense and manipulate the emotions of those around him. Recognizing his high rank in the army, Maria decided to change him into a vampire to help her claim territory in Monterrey. Jasper's responsibility was to train young vampires and then kill them when they were no longer useful (about a year after their transformation- when their strength began to fade).
After about a century of this, he grew weary of the lifestyle and joined an old friend, Peter, and his mate Charlotte. Jasper left the two eventually, not wanting to feed on humans because he could feel his prey's emotions as they died. Alice, foreseeing that they would be together, met him in a half-empty diner in Philadelphia, and together they sought out the Cullen family. During their companionship and travels, they soon fell in love and were later married.
In Twilight, Jasper accompanies Alice and Bella when they are hiding from James. He uses his ability to calm Bella when she is stressed or afraid.
In New Moon, he attacks Bella after she gets a paper cut, prompting the Cullen family to leave Forks to protect Bella.
In Eclipse, he trains his family and the wolf pack to fight newborn vampires. his history becomes known.
In Breaking Dawn, Jasper runs away with Alice to find witnesses for the confrontation with the Volturi. They find his old friends, the nomads Peter and Charlotte, and Nahuel, the half vampire half human boy. You also find out that he is the one that gets the family their drivers licenses and passports from J Jenks, who is also a little scared of 'Mr. Jasper'.
He is described as tall with honey blond hair, and is muscular but lean and is physically 20 years old. His body is covered with crescent-shaped scars, after the many years of fighting and training newborn vampires. Due to his past, where he was able to feed on humans whenever he chose, Jasper lacks strong self-control.