Ash's Pikachu (Master)

Ash's Pikachu is the first Pokémon obtained by Ash Ketchum, and is his main Pokémon.

Kanto
Ash had waited last for his starter Pokémon, and Professor Oak told him he didn't have one. He then remembered he had a Pikachu, which Ash took immediately. Pikachu had at first disobeyed Ash and didn't listen to his commands, but when Ash protected him from a flock of Spearow, Pikachu had gained trust in him.

Pikachu would be used to capture a Caterpie and then his Pidgeotto. Pikachu was then used to battle Brock's Onix, but lost badly. Pikachu had then been taught the Iron Tail move by Flint, and while unable to use it properly, managed to defeat Brock's Onix.

Pikachu didn't battle Misty due to being her friend, but was later on used to battle Lt. Surge. Lt. Surge's Raichu defeated Pikachu, and at the Pokémon Center was given the choice to evolve but didn't. Pikachu trained with Ash and Ritchie, and defeated Ritchie's Fearow but then lost to his Charmander, Zippo. Pikachu ended up defeating Lt. Surge's Raichu and proved that evolution doesn't equate to strength.

Pikachu was used often during the Kanto arc, later on being used to battle Sabrina's Kadabra and after a close battle won. He decided to not battle against Koga and Blaine so other Pokémon could train, but was used to battle Giovanni's last Pokémon, a Rhydon, and won with Iron Tail.

Personality and characteristics
Pikachu being Ash's first Pokémon, is his most loyal even though they had a rough start. Ash relies on Pikachu the most, and often uses him for short one-on-one battles. Many trainers point out their bond and say that no one can break it.

Pikachu is also friendly, as seen when he didn't battle Misty since she was his friend. He also proved to take care of Serena's Togepi quite well, and acted as a big brother towards her.

Trivia

 * Pikachu is the first evolved Pokémon Ash obtained.
 * Pikachu is the first Pokémon that disobeyed Ash.