Pikachu is a bright yellow (electric type) rodent pokemon. Its height goes as tall as the knee of a young boy.
Pikachu is Pokemon #25, and second main character within the Pokemon series. Pikachu is Ash's first Pokemon. Ash is the main character in the Pokemon series.
The series begin with the scenes that you see in the Pokemon Blue game for Gameboy, then switches to a real battle of the Pokemon League on Ash's TV. Nidorino and Gengar are battling, while Gengar puts Nidorino to sleep, the trainer withdraws Nidorino and sends out an Onix.
What Ash is watching is supposed to be a presentation of the Pokemon League for those kids who want to become Pokemon masters. His mother walks in and scolds him about being up past his bedtime. Then plays another informative video for him, saying that he needs to get ready for the Pokemon League.
Soon Ash is shown sleeping in his bed, and is dreaming about which Pokemon to choose. While the clock ticks past the time he was supposed to get up! Ash dreams he is throwing a Poke Ball, when in reality it was his Voltorb shaped clock, it breaks into pieces then he wakes up in a fright!
Needless to say he is awful late and runs in his pajamas to Prof. Oak's Lab, only to find Gary (his rival) in a parade with girls celebrating his new Pokemon journey. Gary brags about how he got a great Pokemon, but when Ash asks him to let him see it, he simply says "it is none of your business, if you showed up on time you would have seen I got the best Pokemon from Prof. Oak, it's good to have a grandfather in the Pokemon business isn't it?"
It so happens that Prof. Oak is the grandfather of Gary, and in many occasions in the series has favored his grandson over Ash. But that's another story. Ash was furious about Gary's mockery. Which is followed by Prof. Oak telling Ash he doesn't look ready for a Pokemon, but he looks more like ready for bed.
When Ash is in the Lab, he attempts to get one Pokeball containing one Pokemon out of the three he was offered: Charmander, Squirtle and Bulbasaur. Unfortunately while Prof. Oak is basically making a joke out of the whole situation, he lets Ash know one-by-one, that the Pokemon were taken by someone else who was on time. He enforces that by saying "the early bird gets the worm, or in this case the Pokemon".
I should mention that rubbing it in 4 times total (outside and inside the lab) is a little too much.
Nevertheless, instead of skipping through all of the above mentioned, after he notices Ash has suffered enough; he decides to let Ash have a Pokemon, that according to him has a problem.
Except from the fact that Pikachu is not used to Ash, and its a bit stubborn, there is really nothing wrong with it. As a matter of fact a few minutes later on the same episode Pikachu saves Ash's life by Thunder-shocking some Spearows. Later on, Pikachu saves Ash once again when he stands up to the Fearows and jolts them once more.
The series continue with all kinds of situations in which Ash and Pikachu stick together through thick-n-thin. One of them is an episode later in the series where Pikachu demonstrated how loyal he is to Ash (in the episode "Pikachu's Goodbye") where Pikachu turns down the chance to go back to the wild after commuting with a bunch of Pikachus. Ash and Pikachu's friendship strengthened much more as time went by, and is still to this day by Ash's side.