- "Jasper has the ability to both feel and manipulate the emotions of those around him."
- ―Entry on Jasper's gift[src]
A vampire with this ability can sense the feelings of others and alter them according to his will. With it, Jasper can resolve a tense situation peacefully or, (like Chelsea with her gift) make people coexist more easily, as Jasper did with his newborn army. It can also be used to enlighten someone's mood; Edward described it as a subtle ability, and is very useful in resolving conflict. It is said that Jasper feels horrible because he could feel Edward's negative emotions when he tried to attack Bella in New Moon.
He can also manipulate the level of intensity people feel emotions, as shown when Jasper calmed Bella so much that she fell asleep in Twilight.
Given the way he was able to influence the mood of the Quilette shape-shifters in Eclipse, it is obvious that his power is not limited to humans and vampires alone.
This power can work potently through a mental shield because it physically changes the emotions i.e. manipulating endorphins, adrenaline and pheromones instead of providing a false alarm to the mind.
With practice, there is possibility that he can overload his targets with emotions as a way of attack.
This ability is limited, as shown in Breaking Dawn, when Jasper and Emmett come to Bella's house to take Edward for a "bachelor party". Jasper reassures Bella with his power (or rather takes away her insecurity at the idea), but as they leave, she states she felt her calm slipping away. This shows that the emotion lasts only as long as the user is around. This limitation was also explained in Eclipse, when Jacob told Bella that Jasper got rid of everyone's anger, which did not resurface until after he was gone.
Jasper was quite charismatic as a human - he could make others view things his way. Upon transformation, this charisma changed into a power to physically sense and control other people's emotions.
While serving in Maria's army, Jasper's power was useful in controlling the newborn soldiers and keeping them calm. However, he sensed the pain of every person he killed (human or vampire). Overtime, that pain built until it became unbearable. When he came across Alice in 1948, he felt an unfamiliar emotion for the first time: hope. Her happy feelings impacted him so strongly that he easily reciprocated her love for him. And then, by practicing to live on animal blood, he was freed from the emotions that depressed him.
In Twilight, Jasper feels Bella's distress about James and uses his power to calm her to the point that she feels sleepy.
In the New Moon film, he changes Bella's mood to make her agree to attend her own 18th birthday party that Alice had planned for her. Bella moans at Jasper for the mood change, to which Jasper, Edward and Alice laugh.
In Eclipse, Bella mentioned not being able to tell if Jasper was making the atmosphere tense or whether she was just feeling it. He helped resolve a tight situation caused by Paul, Emmett and Rosalie.
In Breaking Dawn, Jasper is also at Bella's side after she became a newborn, initially to change her mood in case something upset her. Later, he feels her endlessly happy mood, which makes him gravitate toward her without thinking.
- Didyme's happiness induction is very similar to this power, although it was limited to spreading joy while Jasper can inflict any emotion he wishes.
- Chelsea's ability to manipulate relationships bears a resemblance to this power, though she can bind the emotional bonds between people permanently while Jasper can only control the emotions of those around him for as long as he is present.
- Fred's gift of physical repulsion is similar to Jasper's in the sense that he can change the way someone feels, physically. Though, like Didyme, it is limited to only one emotion: repulsion.