Pikachu

Pikachu is Ash's starter Pokemon.

History
Pikachu was born as a Pichu, and sometime before the series began, evolved into Pikachu.

In Kalos, Where Dreams and Adventures Begin!, Ash and Pikachu arrived in Kalos by plane along with Alexa. After being thrown out of theLumiose Gym for not having enough Gym Badges, Ash was saved by Clemont while Pikachu was saved by Bonnie. Bonnie was excited to be holding Pikachu, leading to him shocking her because she was holding him too tightly. After becoming acquainted with their rescuers, Ash challenged Clemont to a battle, which Clemont reluctantly agreed to.

In the battle, Pikachu faced Clemont's Bunnelby. Despite Clemont admitting that he only recently caught Bunnelby, they proved to be a match for Pikachu. The battle was interuppted by Team Rocket, who once again attempted to steal Pikachu for their plans. Ash attempted to stop them, but Jessie had her Wobbuffet reflect Pikachu's attacks with Mirror Coat. Pikachu was saved by a Froakie, who shielded Pikachu with his body at the cost of being injured. With Bunnelby and Froakie's help, Pikachu was used to blast of the villains. Froakie soon collapsed from his injuries, so Ash, Pikachu, and their new friends went to Professor Sycamore to heal Froakie back to health.

In Lumiose City Pursuit!, Ash and Pikachu managed to get Froakie to Professor Sycamore so that he could be healed. When a Garchomp went crazy from one of Team Rocket's devices, Pikachu and Ash pursued her to get the device off and free her. After chasing her to the top of Prism Tower, Pikachu removed the device from Garchomp's neck with Iron Tail. However, due to the fight damaging the tower, the ground Pikachu was standing on broke off, causing him to fall off with Ash immediately leaping in to save his best friend. They were both saved by a Mega Blaziken, who intercepted them in the air and put the two safely back onto the ground.

Trivia

 * Pikachu was the first of  Ash's Pokemon to have it's gender confirmed.
 * Pikachu has a talent for mimicing other pokemon, contorting his face and ears to mimic the appearance of other Pokemon, or even Ash.
 * Pikachu was the first evolved Pokemon Ash ever got.