Pikachu
Pikachu was Ash's very first Pokemon. Pikachu isn't one of the starter Pokemon, but Ash woke up late and all the other Pokemon were already chosen. They didn't get along too well at first. Pikachu was very stubborn and refused to listen to anythnig Ash said. But after Ash save's Pikachu from a flock of angry Spearows, then they become better friends.
Pikachu doesn't live inside it's Pokeball like most Pokemon. It prefers to roam freely. In case you haven't noticed, most Pokemon take on a form of some type of realistic animal. Pikachu takes after the form of a mouse. I must admit, Pikachu is quite cute.