Pikachu was Ash's Starter Pokémon, given to him by Professor Oak. Ash showed up late to pick his first Pokémon so he got stuck with Pikachu. Though Pikachu didn't like Ash at first, Ash demonstrated his willingness to sacrifice himself for Pikachu, and they quickly became friends. Pikachu, who is rather independent in general, believes that he is strong enough without evolving and refuses to evolve into a Raichu even when he was easily beaten by Lt Surge's Raichu. Later in Ash's adventure; in the Sinnoh region, Pikachu was given another chance to evolve after being seriously injured by another Raichu but once again he refused.
Early in the series, Pikachu was given an operation to increase the strength of the electric sacs in his cheeks in order to beat the first gym leader, Brock. Despite this, Pikachu can also get a cold and go out of control when it cannot release its electricity. Most notably, exposure to a magnetic field will cause Pikachu to get a cold. This has been done to Pikachu three times in the series, 2 times by Team Rocket and 1 time by Zekrom: the first time, Pikachu was cured when the extra electricity was used in attacks against a group of Grimer and Muk, and in the second time, Pikachu was inadvertently cured by one of Team Rocket's battery robots, which subsequently overloaded and exploded the last time is when Professor Juniper heals it after Ash's battle with Trip in Unova.
Many of the Pokémon animated shorts with all-Pokémon dialogue have featured Pikachu as the main character, and as the main voice of reason. As such, Pikachu is often a leader, helping to break up conflicts among other Pokémon.
However there are times when Pikachu gets beaten fairly easily. For example he was easily defeated by Lt Surge's Raichu during their first battle, was no match for Sabrina's Kadabra, got knocked out by a single Night Shade from Morty's Gastly, which was Morty's weakest pokemon, was overwhelmed by Fantina's Drifblim despite its type advantage, lost against Norman's Vigoroth in their first battle, landing only one blow in the process and was defeated in many other battles during his journey with Ash. Despite these defeats Pikachu was considered to be one of Ash's most powerful pokemon, alongside Charizard, Sceptile, Infernape and Snorlax.
Pikachu's high level of electrical power has even allowed him to defeat Ground types more than once by using Electric attacks, which normally would do no damage at all. However this must be an error in the writing of the episodes, since Ground types take no damage from Electric type moves in the games, no matter how powerful they are. Whilst travelling through Hoenn, Pikachu learned how to use Iron Tail which has become one of his signature moves, and now means that he's got a technique that will hurt a Ground type, especially if they are also part Rock type.
In BW052: Dazzling the Nimbasa Gym!, he was used in Ash's fourth Gym battle against Elesa's Emolga (however, he had to Thunderbolt Ash beforehand, in frustruation, as Ash did not even consider using Pikachu). Pikachu countered the EleSquirrel's Electro Ball with his own and defeated it with Quick Attack. Against her Tynamo, Pikachu suffered many direct hits, due to Tynamo's amazing speed, until Ash commanded it to use thunderbolt to alter the battlefield (this way Tynamo would be forced to tackle Pikachu head on only). Ash's tactics work and Pikachu used a powerful Iron Tail attack to win the battle, earning Ash the Bolt Badge.
In BW070: An Amazing Aerial Battle!, Pikachu was used in a Gym battle against Skyla's Unfezant and defeated it with Electro Ball. He stayed in the battle to fight Skyla's last Pokémon, Swanna. Pikachu's Thunderbolt and Electro Ball were blocked by Swanna's Aqua Ring, defending against the type advantage Pikachu held over it. However, using the hoops that Skyla had used in her pre-battle show, Pikachu was able to jump with Quick Attack to hit Swanna. Eventually, Swanna was able to hit Pikachu with a powerful combination Hurricane and a brutal Brave Bird, after which Ash deemed Pikachu unable to battle, although he didn't faint, he tried to continue.
In BW086: Rocking the Virbank Gym! (Part 2), Pikachu was the final Pokemon to go up against Garbodor. It was going to use DoubleSlap but Pikachu evades it and hits Garbodor with Quick Attack and then Thunderbolt. When Pikachu tried to use Iron Tail, Garbodor swung its arms upwards, repelling Pikachu and untying the arms. After Pikachu dodges Gunk Shot twice, Garbodor grabbed him and used DoubleSlap hitting Pikachu repeatedly. After this, Garbodor fired another Gunk Shot, this time it hits and it also poisoned Pikachu. To Ash's luck, Pikachu's Static Ability got activated and paralyzed Garbodor after making physical contact with Pikachu. Despite still being able to withstand a Thunderbolt, the Trash Heap Pokémon was unable to take a hit from the following Electro Ball, and was knocked out, making Ash the winner and earning him his eighth and final Unova Badge, the Toxic Badge.
Pikachu is voiced by Ikue Ootani in all versions of the anime, except in early episodes of the English dubbed series, in which Rachael Lillis provided the voice in cases where another character's voice overlaps Pikachu's. A running gag is how he burns all main female character's bike with Thunderbolt, which, in turn results in that girl following Ash, except for Iris following Ash after she and Ash bumped into each other. Pikachu also has a fondness for ketchup, which is seen multiple times in the anime.
When Ash received Pikachu from Professor Oak, his Poké Ball had a lightning bolt on the top part.
Because Pikachu doesn't like being inside his Poké Ball, it hasn't been seen since the first episode. Except for the episode where he first battles Brock for the boulder badge, when Ash tries to call Pikachu back while stuck in Onix's wrap attack; but Onix blocks the return beam.
Ash attempted to evolve Pikachu with the Thunder Stone after he lost to Lt. Surge's Raichu, but Pikachu chose not to evolve because he wanted to prove that he can defeat stronger Pokémon without evolving.
Pikachu occasionally has acted as a responsible leader of the main cast's Pokémon, mainly Ash's Pokémon.
In the season one episode "Showdown at Dark City", Pikachu displayed a strong fondness for ketchup. In fact, when the ketchup bottle he was carrying was smashed, Pikachu was seen crying over the smashed ketchup bottle. This could be an indication on how close Pikachu's relationship with Ash is, since his last name is phonetically similar to ketchup.
In Climbing the Tower of Success, a Thunder Stone was tossed to Ash, thus it almost landed on Pikachu which would cause him to evolve. However, Ash was scared about Pikachu evolving, so he prevented this, which contradicts to the time he offered Pikachu a Thunder Stone to evolve. The most likely reason is that Ash now knows Pikachu doesn't want to evolve (and likes keeping him the way he is and respects his wishes), which is supported in the Diamond and Pearl generation episode Pika and Goliath.
Both Ash's Pikachu and Snorlax are evolved baby Pokémon but in that generation they are the first forms.
Pikachu had a special event for the 4th Generation of Pokémon Games knowing the moves Pikachu had in Sinnoh League Victors.
Ash's Pikachu is finally confirmed male in Where no Togepi has gone before after being hit by Togepi Attract and Jessie Yanmega was confirmed female.
Pikachu seems to have a great dislike, or even hatred, against Team Rocket's Meowth. As after Meowth and his team got away from the failed railroad mission, Pikachu still exerted a lot of anger and Ash had to hold him down. While Meowth was in Ash's group, Pikachu was the one to always be suspicious of Meowth's loyalty.
Ash's Pikachu is the most recognizable Pokémon, therefore being the face of Pokémon.
Some how for every region Ash has traveled through with Pikachu he has always been for the most part surpassed in power by another Pokémon that Ash catches and trains until they fully evolve or learn new moves.
Beginning from Hoenn onward, every time Ash and Pikachu enter a new region, Pikachu has been exposed to high levels of electricity or magnetism, before or after arrival, and loses control of its own electricity, becomes confused, and runs away, forcing Ash to chase after it.
Pikachu has had many moves over the seasons, but the amount it now actively uses has been lowered to only four.
Despite Pikachu's absence from the Unova Dex and not being native to the Unova region, a Trainer's Pikachu can be seen in Castelia City, and another Pikachu is depicted on a billboard. Also, there are several Pikachu-shaped hedges in Striaton City.