"This Pokémon generates electricity and channels it into its legs to keep them going strong. Boltund can run nonstop for three full days. It sends electricity through its legs to boost their strength. Running at top speed, it easily breaks 50 mph."
Yamper first appeared in A Sword-id Meeting when Lexi arrived in Postwick, being looked after by Sonia. However, it appeared that Sonia was unable to take care of Yamper due to his excitable nature. When Lexi had a panic attack and ran away into a woodland area, Yamper followed her in order to make sure she was okay. Yamper helped Lexi calm down, and was then used in battle against Zacian. Sonia realised that Yamper was following Lexi around, and asked her to have him on her team. She accepted, and Yamper joined her.
Following this, Yamper remained outside its Pokéball for some time. Yamper continued to grow as a Pokémon and a companion to Lexi. He would often jump into action at the drop of a command to help her, and be a strong competitor. He relished in praise, and often got so excitable at the thought of it. He was also fiercly protective, trying to appear intimidating to anyone he deemed a threat.
Yamper often struggled with appearing "dumb" to other Trainers, which would result in him trying too hard to be mean spirited, which often left him unfocused and susceptible to attacks. As time progressed, Lexi taught him that all he needed to be was himself, as being intimidating would not prevail him in battle. Once Yamper begun to accept this, he realised that he can use his nature to his advantage, often to outsmart his opponent. This also led him to be open to evolution; during Lexi's battle with A, he fought against intimidating Pokémon; once he mastered his battling techniques without being intimidating, he evolved into Boltund.
Once it evolved, it remained in its Pokéball. It became a powerhouse and was very focused in battle. Boltund was used regularly in battles and whenever he appeared, was very excitable still but was able to control himself easily. Even despite this, he remained very affectionate towards Lexi whenever he wasn't battling.
As a Yamper, he is an exciteable fun loving Pokémon, who always has a smile on his face, for which Sonia describes him as "dumb looking". Despite this, Yamper is very motivated to battle, alongside making friends with other Pokémon and Trainers at any moment.
Boltund has incredible stamina, an ability that allows him to keep up with high-intensity activities. At top speed, Boltund can easily break speeds of 50mph that he can keep up non-stop for three days.