In Avatar, Tayäìo is the sister of Tsu'Tey, the Omatikaya clan's next eyktan, who learned English from Grace Augustine's school before it was shut down due to the clan's strained relationship towards the 'aliens' known as humans from Earth, those who represented the RDA. It would allow Tayäìo to do more as she grows from child to adulthood, becoming an ikran makto (rider of Banshee), a good hunter, and not understanding much as to why her brother spews so much hatred towards the RDA. She remembered their teacher, Grace, being very kind and gentle towards them as children.

When it was announced by Eytukan and Mo'at that Tsu'Tey and Neytiri were to become the next Tsahìk and Eyktan, she was happy for her brother, it would also mean she would be paired with a tutan, she would do what makes her brother happy as her parents were already gone, having gone back to Eywa, but she would disobey his orders by getting close to Hell's Gate to watch the Tawtute go about their day.

In New Moon, the Cullens throw an 18th birthday party for Bella. In the movie adaptation, Jasper uses his empathic power to alter Bella's mood to make her feel comfortable enough to attend her birthday party.

While opening a gift, she cuts her finger lightly and starts bleeding. Since Jasper is the newest vegetarian vampire in the family, he is overcome by his bloodlust and tries to attack Bella, only to be stopped by Edward, though Emmett puts Jasper in an unbreakable grip. Edward pushes Bella away and she is not bitten, but is injured further when she crashes into broken glass, requiring stitches. After Jasper nearly bites Bella, he and his family move away from Forks in an effort to protect her from their world, demanded by Edward. After they moved up to Cornell, Jasper started studying philosophy.

When Alice has a vision of Bella jumping off a cliff, she rushes back to Forks alone. However, when Rosalie tells Edward about this incident, he is led to believe that Bella has died and travels to Volterra, Italy for a suicidal plan. Jasper is warned by Alice to stay put while she and Bella rush to Italy to stop him. After they successfully saved Edward from committing suicide, Jasper moves back to Forks with the rest of the Cullens.

At the end of New Moon, Bella gathers the family to vote whether for her to become a vampire or stay human. Jasper casts his vote to make Bella a vampire.

The events in Eclipse focus more on the newborn attacks caused by Victoria's creation of her newborn army in Seattle. Bella is introduced to the term "newborn" for the first time, and Jasper, who was involved with newborns in his early life as a vampire, explains them to Bella. This is how she learns about some of the issues she will have to deal with when she herself becomes a newborn. After following news coverage of several unexplained supernatural murders and disappearances, Jasper is the first to suspect that someone is creating an army of newborn vampires, but dismisses it for awhile because he thinks he has read the signs wrong, until his suspicions are proven correct.

Since Jasper has a great deal of past experience with the newborns, including combat knowledge, the Cullens turn to him for guidance in preparation for the upcoming battle against Victoria and her savage army. He helps train both the Cullens and the Quileute wolves by putting on sparring demonstrations, and pointing out the strengths and weaknesses of newborns for both groups. Jasper's military background helps him coordinate a strategy between the Cullens and Quileutes, which ends up working perfectly. Jasper, however, attempts to be everywhere at once during the battle, helping his family and the wolves fight while protecting Alice, and ends up getting bitten by a few newborns in the process.

Jasper also tells Bella about his early life in the Confederate army, how he was turned, his involvement with Maria, Nettie and Lucy, and his role in helping to form Maria's newborn army, as well as how he met Alice.

In Breaking Dawn, Jasper feels Bella's nervousness about her marriage to Edward and is able to calm her down for a short period of time before going on a private bachelor "party" with Edward and Emmett. On the day of the wedding, he is assigned with picking up Renée and Phil from their hotel.

When Bella became pregnant with a half-vampire child, he is one of the family members who wish to abort the child for her health, but because some of the other family members are against the idea, he decides to remain neutral. With the Uley pack threatening to kill Bella and possibly some of his family, Jasper guards around the house. It does not take long before he learns to love his new niece, "Renesmee Cullen". He is also seen active in the fight with the wolves while guarding the house from their assault in Breaking Dawn - Part 1.

While Bella is a newly turned vampire, Jasper is always standing beside her to make sure she wouldn't lose control and hurt anyone. He is shocked by the fact that she is able to retain self-control, something that most newborns aren't capable of doing. This causes him some dismay over his own struggles with self-control, even as a mature vampire, and also prompts him to re-evaluate his view on newborns.

When the Cullens realize the Volturi are coming as a result of Irina's wrongful accusation about Renesmee, Jasper flees with Alice to South America in hope of finding someone more connected to his adoptive niece's species. It is revealed that Jasper has a secret business relationship with a lawyer in Seattle named J. Jenks after Alice leaves Bella clues to find him. Jasper has been employing him and his former partner to make illegal documents for the Cullens whenever they move to a new place, and this connection allows Bella to forge fake ID's, passports, and a driver's license made for Renesmee and Jacob. Jasper and Alice's search eventually leads them to South America where they find a human/vampire hybrid named Nahuel. They bring him and his biological aunt, a vampire named Huilen, back to Forks in time to prevent a fight between the Cullens, the other vampires (nomads and covens), the wolf packs, and the Volturi.

In the movie, Alice shows a vision of an almost-inevitable battle erupting between the Cullens and the Volturi: in it, Jasper tries to defeat Demetri, but is halted by Jane's power and gets killed, leaving Alice screaming in pain and later goes after Jane for revenge. However, by showing Aro this vision (which includes the demise of his entire coven), he refrains from beginning the battle.

After the peaceful outcome of the situation, Jasper and Alice are welcomed back into the Cullen family. Bella decides to take over Jasper's relationship with J. to save him heart attacks since J. found Jasper to be 'intimidating'. Also, Jasper has a brief conversation with friends Peter and Charlotte before they departed.