Ash's Pikachu (Aura)

Ash's Pikachu is the first Pokémon obtained by Ash Calem Ketchum on his journey as a Pokémon Trainer.

Prior to series
As a Pichu, he was found under Ash's house. When he saw that the small Pokémon was starving, Ash chose to feed him. Seeing how Ash was friendly with Pichu, Delia decided it would be alright for her son to keep the Mouse Pokémon. Some years later, Pichu evolved into Pikachu.

Kanto
While Leaf Green chose her Bulbasaur, Serena Yvonne Gabena chose her Charmander (whom she later nicknamed Charla), and Gary Oak chose his Squirtle, Ash chose to start with Pikachu, to which the Mouse Pokémon was all too happy to agree with. After Ash caused a flock of Spearow to attack them, Pikachu saw that Ash generally cared for him and let loose a massive Thunderbolt on the Spearow, scarring them off.

Upon arriving in the Viridian City Pokémon Center, Ash and Serena took Pikachu to be treated. It was here that they would meet Jacob McKnight and Sally Pisen from the Unova region, and defend the Pokémon Center from Team Rocket Agents Jessie and James.

Jacob asked if he and Sally would be allowed to join Ash and Serena, to which the young trainers happily agreed to. While in Viridian Forest, Ash managed to capture a Caterpie, which would later evolve into Metapod and then into Butterfree, and with Pikachu, managed to catch a Pidgeotto.

After Jacob gave Ash the TM for learning Iron Tail, Pikachu was used against the Pewter City Gym Leader Brock's Onix after Pidgeotto weakened him. Though Onix managed to put up a good fight, Pikachu came out on top.

At Mt. Moon, Pikachu was used along with Butterfree and Pidgeotto against the Pokémon used by Jessie, James, and the Rocket Grunts. Pikachu was then used in Ash's Gym Battle against the Cerulean City Gym Leader Misty. After her Starmie defeated Butterfree, Pikachu stepped in and won against the Mysterious Pokémon.

At the Hidden Village, Ash had Pikachu battle Bulbasaur, who soon joined the team, and later stood up for Charmander against Damian, along with Smaug. Pikachu was later instrumental in helping the people on aboard the S.S. Anne against Team Rocket, and welcomed Squirtle to the group. Pikachu would then later be used in Ash's gym battle against Vermilion City Gym Leader Lt. Surge, where he squared off against Lt. Surge's signature Pokémon, Raichu. On the advice of Jacob, Ash had Pikachu use his speed against Raichu, and won.

Following the events of rescuing a Pink Butterfree from a poacher and liberating the Pokémon Tower in Lavender Town from Team Rocket Grunts, in which Pikachu would become a big brother figure for an orphaned Cubone, Pikachu soon received a sickness from an electric overload, resulting in a cold. This resulted in a Magnemite following him around. As soon as Pikachu was better, it stopped following him around.

Pikachu would then be used in Ash's Gym Battle with the Celadon City Gym Leader Erika, where he fought against her signature Pokémon, Vileplume, and won. Pikachu was later present when Ash and his friends went to liberate the Silph Company of Team Rocket. Pikachu was later injured while fighting Jessie's Arbok and James' Weezing.

Pikachu was later used in Ash's Gym Battle with the Saffron City Gym Leader Sabrina. After Haunter took out her Mr. Mime, Pikachu went up against Kadabra. Kadabra was quite a challenge, but thanks to an Aura power boost from Ash, Pikachu came out on top and won.

Pikachu was among many of Ash's Pokémon who enjoyed a slice of birthday cake for Ash's 11th birthday. Later on, Pikachu was among the many Pokémon who helped keep Ash and Serena warm in a cave during a snowstorm. During the Gym Battles against the Fuchsia City Gym Leader Janine and the Cinnabar Island Gym Leader Blaine, Pikachu sat them out to let other Pokémon like Haunter, Muk, and Hitmonchan have their moments to shine.

Pikachu would become a big brother again for a newly hatched Togepi. Pikachu would later be the last Pokémon Ash would use against the Viridian City Gym Leader and Team Rocket Boss Giovanni's Rhyperior. Thanks to an Aura boost from Ash, and a Psychic boost from Serena, Pikachu eventually came out on top, earning Ash his eighth and final Kanto region gym badge.

During the events on New Island, Pikachu followed Smaug's example and chose not to fight his clone, Pikachutwo. When Ash stepped in between a blast of psychic energy from Mew and Mewtwo, Serena and Pikachu ran up to Ash, trying to get him to wake up. Pikachu's electrical attacks did no good as it looked like Ash was now dead. But thanks to the tears of all the Pokémon around them, Ash was revived.

At Indigo Plateau, Ash used Pikachu in a practice battle against Jeanette's Scyther in Bulbasaur's place, and won. But when going against Jeanette's Bellsprout, Pikachu lost and Dratini went in his place and won. Pikachu would later be used in when Ash went up against Ritchie, and managed to win while learning Wild Charge, but also knocking himself out at the same time.

Orange Islands
Pikachu accompanied Ash and his friends on their journey to the Orange Archipelago. On the way, Pikachu fend off the flock of Spearow from before that were now being led by a Fearow that had since evolved from one of the Spearow, which Ash hit when they started their journey. Upon arriving on Valenica Island and meeting Professor Ivy and getting the GS Ball, Pikachu said goodbye to Smaug as he and Jacob had to return to Unova to attend to Jacob's ailing little sister.

After arriving on Tangelo Island, Ash and his friends would later welcome Tracey Sketchit and a Lapras into their family. Later, on Mandarin Island, Pikachu was a part of a Pokévision video with Ash. On Navel Island, Pikachu participated in Ash's gym battle with the Navel Island Gym Leader Danny, along with Charizard and Primeape in a rotation battle. Pikachu went against Nidoqueen first, then Electrode before switching with Charizard. After Primeape had a turn against Machamp, Pikachu went against the Fighting-type next and managed to use Wild Charge to beat Machamp. Then Pikachu let Charizard have a turn against Electrode. Pikachu next went against Nidoqueen before allowing Charizard to have a go.

Pikachu would later be used in a target game Trovita Island. During Ash and his friends' adventures around the Seven Grapefruit Islands, Pikachu helped to safe the grapefruits from a notoriously hungry Snorlax, which Ash added to the team. On Kumquat Island, Pikachu helped to encourage Dratini in Ash's gym battle with the Kumquat Island Gym Leader Luana.

The group later celebrated Ash's 12th birthday on Shamouti Island, and later took part in helping Ash and friends save the world by helping Lugia to tame the beast of the sea. After that, Ash and his friends arrived on Pummelo Island, and Pikachu did some training with Bulbasaur, so that they could be ready for when Ash had his battle with the Pummelo Island Stadium Supreme Gym Leader Drake. Though Pikachu lost against Drake's Ditto, Pikachu did weaken Ditto enough for Primeape to beat it. Pikachu would later help comfort Ash when he learned that the previous winner of the Orange League was his own half-brother, Silver.

Upon returning to Pallet Town, Ash, Pikachu and the others got reacquainted with Jacob and Sally, and met Jacob's little sister Mia McKnight for the first time. Pikachu became fond of the autistic girl, but nearly unleashed a Thunderbolt on her as she hugged him too tightly, but Ash pulled Pikachu away from Mia and took the blow for her. Ash would later use Pikachu against Gary's Eevee and won.

Johto
In Cherrygrove City, Pikachu was used to help deal with a swarm of Heracross that were eating the sap of the cherry blossom trees. Ash had him battle the leader of the swarm with Gligar's help, and learned Electro Ball in the process.

Pikachu would later be used to try and help Ash capture a Chikorita, though she put up a good fight. Ash then recalled Pikachu and sent out Snorlax. Pikachu took a back seat during Ash's gym battles against Falkner and Bugsy, gave Pokémon Hunter Rico quite a shock with Thunderbolt, and would help in catching a Noctowl in Ilex Forest. After this, Pikachu was used to battle the leader of a pack of Houndour, but it was interrupted by the howl of their sick pack member.

Kalos
Pikachu was among the Pokémon who participated in the final battle against Team Flare.

Personality and characteristics
Pikachu is among Ash's most loyal Pokémon.