A solo picture of Pikachu.

is the one of the most well known Pokemon in America and Japan. It is number 25 in the National Pokedex and is Ash's favorite and first Pokemon, as well as his best friend. Pikachu first appeared in the first Pokemon episode and has been in every episode since. If Ash had never overslept the morning he became a trainer, he would never have Pikachu. If Ash did not oversleep, he would have chosen Squrtle, Bulbasaur, or Charmander as a starter. Pikachu is also your starter in Pokemon Yellow Version. On other games, you can catch a wild Pikachu in the wild. He is a very common and popular Pokemon. Pikachu appears in all the generations, most likely because he is a very common Pokemon that most people know even, even if they don't play Pokemon.
It's hard to find Pikachu in most Pokemon games, he is hard to find. Pikachu evolves into Raichu.