Ash's Pikachu

Ash's Pikachu is the first Pokémon that Ash ever received and since then he's been with him since the beginning of their journey.

Biography
Pikachu was Ash's Starter Pokémon, given to him by Professor Oak. Ash showed up late to picking 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 became fast 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. Several years later, 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 twice in the series, both times by Team Rocket: 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.

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.

Pikachu is one of Ash's most powerful pokemon being able to defeat psuedo-legendary pokemon such as Metagross and Dragonite and even actual legendary pokemon such as Regice. He was also capable of fighting Tobias' powerful Latios to a draw, despite the fact that Electric type moves were not very effective against Dragon types. Pikachu was used in many of Ash's Gym, Battle Frontier and League battles, being able to defeat Juan's Milotic, Lt. Surge's Raichu (although this was during their rematch), Clair's Gyarados, Crasher Wake's Gyarados, Volkner's Electivire, Roxanne's Nosepass and her Geodude, Brock's Geodude and Onix, Roark's Onix, Brandon's Regice and countless other opponents.

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 five most powerful pokemon, the other four being 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 BW020, a group of Ducklett dropped an umbrella on Pikachu as it was about to use Thunderbolt, which affected his electric attacks as a result. Soon after, Pikachu learned Electro Ball and defeated the Ducklett with this move. It then battled the Sunglasses Sandile. During the battle, Sandile evolved into Krokorok and fired a Stone Edge at Pikachu, who used Electro Ball and sent the move back at Krokorok, blasting it off in the process.

In BW043, he was used to battle Iris' Excadrill during the final round in the Don Battle Tournament at Nimbasa Town. Because of a weakness to ground-types, he was unable to defeat the Subterrean Pokemon and lost.

In BW052, he was used in Ash's fourth Gym battle against Elesa's Emolga. he countered the EleSquirrel's Electro Ball with his own and defeated it with Quick Attack. Against her Tynamo, it used a powerful Iron Tail attack to win the battle, earning Ash the Bolt 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.

Trivia

 * Ash attempted to evolve Pikachu with the Thunder Stone after he lost to Lt. Surge's Raichu, but he chose not to evolve because he wanted to prove that he can defeat stronger Pokémon without the need to evolve.
 * 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 the Season One episode "Ash Catches a Pokémon", when Team Rocket was trying to steal Pikachu, Meowth accidently says that Ash's Pikachu's power exceeds the power level he would possess as a Raichu.
 * 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 Pikachu is finally confirmed male in Where no Togepi has gone before after being hit by Togepi Attract and Jessie Yanmega was confirmed female.
 * Pickachu 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.