Ash's Pikachu (Aura)

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

Kanto
Pikachu was the only Pokémon Professor Oak had left, after Leaf Green chose her Bulbasaur, Serena Yvonne Gabena chose her Charmander (whom she later nicknamed Charla), and Gary Oak chose his Squirtle. When Ash first met Pikachu, the rodent Pokémon wasn't sure if he wanted to be his Pokémon. 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 Brock's Onix after Pidgeotto weakened him. Pikachu used Agility to dodge Onix's Rock Throw, jumping on the rocks to reach up to Onix's head and used Iron Tail three times. Before he could do a fourth Iron Tail, Onix swatted Pikachu off with his tail. Onix then trapped Pikachu in Rock Tomb, but Pikachu used Iron Tail to break himself free. Before Onix could use Rock Tomb again, Pikachu used Agility to get away, forcing Onix to use Rock Throw. Ash then had Pikachu use Iron Tail on the rocks so as to aim them at Onix, which worked and left Onix dazed for a bit. Then Pikachu used Quick Attack, following it up with Iron Tail, which was enough to defeat Onix and earn Ash his first Kanto badge.

At Mt. Moon, Pikachu was used along with Butterfree and Pideotto 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. Starmie managed to dodge Pikachu's Thunder Shock and used Tackle, causing Pikachu to fall into the pool. Then Starmie used Water Gun and Tackle. Pikachu tried Thunder Shock again, but Starmie dodged it. Then Ash had Pikachu use Iron Tail, which managed to stun Starmie, leaving Pikachu free to use Thunder Shock, which morphed into a Thunderbolt, securing a win for Ash and another Kanto badge.

At the Hidden Village, Ash had Pikachu battle Bulbasaur, who soon joined the team. Pikachu was later instrumental in helping the people on aboard the S.S. Anne against Team Rocket. 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. Pikachu used Quick Attack, but Raichu grabbed him by the tail and used Thunder Shock, and followed up with a lot of Body Slams. But Pikachu managed to get away and used Agility to escape Raichu's Body Slams. Then Raichu used Thunderbolt, but Pikachu stood on his tail to avoid it. Raichu then began to run out of power as Pikachu used Quick Attack, then followed it up with Slam, and finally Iron Tail, earning Ash his third Kanto badge.

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. Pikachu first used Thunderbolt, but Vileplume retaliated with Petal Dance. Pikachu then used Agility to dodge Vileplume's Stun Spore and then used Iron Tail. Vileplume then used Acid but Pikachu managed to use Agility to dodge and followed it up with a Quick Attack until Acid took effect. Vileplume then charged up for a Solar Beam. Pikachu prepared to use Iron Tail, but Vileplume used Stun Spore while continuing to charge her Solar Beam. But Pikachu refused to give up, and managed to counter the Solar Beam with his Thunderbolt, resulting in Ash winning the match and his fourth badge.

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. Pikachu first used Agility, while Kadabra used Disable. Pikachu tore his eyes away from Kadabra so Disable wouldn't have any effect, but Kadabra then used Teleport and appeared right in front of Pikachu. Pikachu then used Quick Attack to ram himself into Kadabra's midsection, causing the Psi Pokémon to stumble backwards. Kadabra used Teleport again, but as Pikachu was about to use Quick Attack, Kadrabra used Disable and then Confusion to contort Pikachu's body into many kinds of awkward positions. Suddenly, a mysterious power inside Ash, later revealed to be Ash's Aura, kicked in and connected with Pikachu who broke free of Kadabra's psychic control. Kadabra used Psychic, but Pikachu leapt out of the way, and used Iron Tail at Kadabra's midsection, causing the Psi Pokémon to crash against the gym wall. Pikachu then tried following it up with Thunderbolt, while Kadabra tried to counter with Psybeam, which caused an explosion. Then Pikachu used Agility, following it up with Quick Attack, giving Kadabra little time to dodge or Teleport. Kadabra then grabbed Pikachu by the neck and emitted a Psychic attack, but Ash and Pikachu refused to give up. Pikachu then used Thunder for the first time, charring Kadabra completely. Then Pikachu followed it up with one Iron Tail after another until Kadabra was finally defeated, earning Ash a win as well as his fifth badge.

Personality and characteristics
After Ash gained Pikachu's loyalty, Pikachu became extremely loyal to Ash and has maintained this loyalty, and while he is not always immediately trusting of strangers, he is generally friendly to humans and Pokémon alike. He acted as a guardian and big brother to Serena's Togepi, and has been seen mediating and even breaking up fights among other Pokémon. During the events on New Island, Pikachu, Smaug, Ash's Dratini, and Dratini's clone were the only Pokémon not to participate in the giant battle; Pikachu stood motionless as his clone repeatedly slapped his face. Contrary to his distrusting nature, Pikachu took a liking to Serena very early, and has retained this liking to this day.

On several occasions, Pikachu has developed an "electric flu" causing him to become overcharged and extremely powerful. During these instances, Pikachu will sometimes accidentally let off electric outbursts which can be set off on anyone, including Ash. In all of these cases, Pikachu did not mean harm upon anyone.

Pikachu has a special fondness for ketchup, but he will eat and enjoy almost any other human or Pokémon food.

Pikachu, like his Trainer, is always happy to battle. He often tries his very best, but when he loses, he doesn't complain or feel bad about himself, especially when Ash reassures him. Ash and Pikachu are almost identical personality-wise, and thus are the perfect match for one another.

In general, Pikachu as a species is not particularly powerful, but Ash's Pikachu is an anomaly, overcoming odds in battle numerous times. Jacob McKnight even noted that Pikachu's power "far exceeds his evolutionary level". Ash's Pikachu has also defeated very powerful Pokémon that Ash's other Pokémon were unable to, such as Rhyperior, Metagross, and Mega Lucario. He has helped Ash win countless battles and Gym and League matches.

However, at times, Pikachu has lived up to his species' frail and brittle strengths, particularly in Gym battles where he was defeated with relative ease by his opponent; for example, he was overpowered very easily by Bugsy's Scyther, Whitney's Miltank, Chuck's Poliwrath, Roark's Rampardos, Fantina's Drifblim, and Viola's Surskit, despite having a type advantage with Scyther, Poliwrath, Drifblim, and Surskit. He also drew against Norman's Vigoroth, Winona's Pelipper, Cress' Panpour, and struggled against Crasher Wake's Floatzel.

Even with his high level of electrical power, Pikachu has not stopped learning new attacks. Upon learning that Brock, the Pewter City Gym Leader used Rock types, Ash had Pikachu learn Iron Tail with TM23 given to him by Jacob. This move allowed Pikachu to defeat Brock's Onix and earn Ash his first Kanto Badge. In addition to giving Pikachu an advantage over Rock types, Iron Tail has allowed Pikachu to defeat many powerful opponents. Later on in their journey, Ash ordered Pikachu to do a Quick Attack. Much to the surprise of Ash and his friends, the Quick Attack morphed into Volt Tackle, and he later learned Electro Ball. Not only that, Pikachu has used his ability, Static, a few times in battle.