Pikachu was Ash Ketchum's first Pokémon that he received from Professor Oak. It is also the mascot for the entire Pokémon franchise, and probably the most widely recognized Pokémon.
Pikachu is Ash's constant companion, rarely leaving his side. Pikachu is also known for disliking traveling inside of a Poké Ball and usually rides on Ash's shoulder.
He is considered to be one of Ash's most powerful Pokémon, helping Ash win countless battles. Pikachu is his most often used Pokémon, partly due to the fact that he consistently stays on Ash's shoulder and is ready to battle at a moments notice. He is also the only Pokémon that Ash brought with him to the Hoenn region and was supposed to be the only one to be brought to the Sinnoh region.
Ever since Team Rocket's introduction, they have tried to capture Pikachu in nearly every episode. They think that Pikachu is an extremely rare specimen and their Boss would reward them greatly for its capture. Even though they have come close several times, Pikachu has always been able to slip through their grasp.
Pikachu is also the only Pokémon that has speech patterns that allow him to be understood by more than just Ash.
