On his first day as a Pokémon Trainer, Ash learned from Professor Oak that the Kantostarter Pokémon, Bulbasaur, Charmander, and Squirtle, had already been taken by other Trainers from Pallet Town. Professor Oak then revealed there was one Pokémon left to give away. This turned out to be Pikachu, who acted very stubborn and showed no particular interest in Ash. Despite this, Ash chose Pikachu to be his partner, and together, they set out on their journey.
Pikachu initially gave the inexperienced Trainer a hard time, refusing to listen to his commands. It was not until Ash accidentally angered a flock of wildSpearow and Ash protected Pikachu from them that they started to bond. After this, they encountered Ho-Oh and Ash vowed to encounter Ho-Oh again someday with Pikachu. Pikachu would subsequently become a trusted partner for Ash, helping him catch a Caterpie and win Badges. Ash and Pikachu eventually met two other Trainers, named Verity and Sorrel, and found another friend in an abandoned Charmander.
After losing a battle against a Trainer named Cross, Ash started to question himself and his Pokémon, and even wished he had started out with a Bulbasaur or Squirtle instead of Pikachu. Luckily, a mysterious Pokémon called Marshadow helped Ash see the error in his new ways by pulling him into a dream where Pikachu was completely absent. Ash apologized to Pikachu and they continued their journey towards the Raizen Mountains to see Ho-Oh again.
Pikachu and Ash
Atop the Raizen Mountains at Mount Tensei, Ash's Rainbow Wing became corrupted when Cross appeared and placed it on Rainbow Rock with his evil heart. Marshadow, in turn, became corrupted by the Rainbow Wing and managed to possess a group of nearby Pokémon. Ash used Pikachu to battle against Marshadow, but the Gloomdweller Pokémon proved to be too powerful and it eventually won, but not without exhausting itself in the process. Ash grabbed the injured Pikachu to keep out of harm from Marshadow's army, only for the two to run into a dead end.
When Ash attempted to get Pikachu to return to his Poké Ball for safety, it refused. Pikachu, appearing to speak, revealed to Ash that it wished to stay by his side forever. Mirroring the start of their journey, Ash tried to protect Pikachu by standing in the way of the attacks. He eventually managed to recall Pikachu into his Poké Ball. Taking the full blast, Ash's body started to disappear, with only his consciousness surviving in a spirit world. Pikachu took hold of Ash's hat and started to cry, with the corrupted Rainbow Wing withering away, freeing Marshadow and the other Pokémon of its control.
Ash could hear Pikachu's cries in the spirit world and ran towards his voice. This caused his body to regenerate in the real world. Noticing this, Pikachu ran towards Ash and reunited with him, fully reviving him. After placing a new Rainbow Wing atop Rainbow Rock with his pure heart, Ho-Oh finally appeared. Ash and Pikachu had a fierce battle with Ho-Oh until the sun set. Some time after, Ash and Pikachu said their goodbyes to Verity and Sorrel, who continued on their own journeys. Pikachu continued to travel with Ash, aiding him in his dream to become a Pokémon Master.
The Power of Us
This section is incomplete. Please feel free to edit this section to add missing information and complete it. Reason: Expansion on M21 role.
Pikachu was later used by Risa, a beginning Trainer who lacked Pokémon of her own, to battle a wild Eevee, which she was ultimately able to catch.
Personality and characteristics
Pikachu's imitation skills
Much like the Pikachu of the main series, Pikachu was initially difficult and uninterested in Ash during the start of their journey, but became extremely loyal to him after he protected him from a flock of Spearow. He also took on Ash's interest and enthusiasm in battling. In addition, the two had a very close bond, to the point where Pikachu issued a massive Thunderbolt in outrage at Ash's death, and then started to cry.
Like the Pikachu from the main series, Pikachu has demonstrated an uncanny ability to imitate other Pokémon, as seen when he imitated a Trainer's Ivysaur.
Following the six cap-wearing event Pikachu distributed in celebration of both the twentieth Pokémon movie and the 20th anniversary of the Pokémon anime (all of which were based on Ash’s Pikachu from the main anime), a seventh Cap Pikachu, based on this Pikachu and wearing the hat Ash wears in the twentieth movie, debuted in Pokémon Ultra Sun and Ultra Moon. Like his predecessors, this Pikachu is always male and cannot evolve. His moveset is identical to the one he had in the movie in which he appeared, and his level corrusponds to the number of said movie. Also like the others, he has access to the exclusive Z-Crystal, Pikashunium Z, and its Z-Move, 10,000,000 Volt Thunderbolt.
In Japan, it was distributed via QR Code on a special Pokémon Ga-Olé disc. In the Americas, Europe, Australia, and New Zealand, it was distributed via a QR Code that was given to those who attend screenings of the 20th movie. Unlike the other six, this Pikachu is only available in Ultra Sun and Ultra Moon, and cannot be transferred to Sun and Moon.