Pickachu was Ash's first Pokemon when he started his quest to become a Pokemon Master. At first Pikachu didn't like Ash and even attacked him. Pikachu was disobedient and wouldn't even get into his poke-ball. Eventually Ash saved Pikachu from a Spearo attack and Pikachu stops giving Ash a hard time though he still prefers to stay out of his poke-ball. Ash used his Pikachu in his first gym battle defeating Brock's Onix by triggering the sprinkler system causing the Onix to get wet with water one of it's weaknesses. Pikachu is abnormally powerful for it's species being able to defeat powerful Pokemon such as Dragonite and Metagross and he has helped Ash win many of his gym battles. Pikachu acted like a guardian and big brother to Misty's Togepi and often tries to break up fights between other Pokemon. Pikachu is also the only one of Ash's pokemon to travel with him to every single region and therefore is his most used and strongest.