Squirtle was the sixth Pokémon Ash caught. He was originally the leader of the Squirtle Squad but took a liking to Ash and decided to travel with him. Squirtle was one of Ash's most commonly used Pokemon throughout his travels in Kanto and the Orange Islands and proved to be a powerful fighter despite his small size. Squirtle was good friends with Ash's Bulbasaur and like Bulbasaur he has shown no interest in evolving despite the fact that he was certainly strong enough to have evolved. Squirtle kept the glasses he had while he was a member of the Squirtle Squad in his shell but rarely wore them.
Shortly after their adventures in the Orange Islands, Ash asked Squirtle to go back to the Squirtle Squad to help them stop fires.
Squirtle returned when Ash's battled Macey in the Silver Conference, defeating Macey's Electabuzz and Quilava. Squirtle's latest appearance was when he joined Ash's team, along with Bulbasaur and Charizard in order to battle against Brandon. Squirtle defeated Brandon's Ninjask but was defeated by Solrock. After the battle, Squirtle returned to the Squirtle Squad and has not been seen since.
It is set to reappear in the Pikachu short airing with the 15th Pokemon movie. It will also reappear along with all of Ash's Pokemon in an upcoming episode of Best Wishes Season 2, where it pays Ash a visit in Pallet Town along with Ash's Primeape and Ash's Charizard.
Squirtle is Ash's first Pokémon to have a trademark item; his sunglasses, Sceptile during all of its evolutionary phases; the stick in its mouth, and the thrid, Krokorok coincidentally also wears sunglasses.
It is the first of Ash's Pokémon to be a leader of a gang.
It is the only one out of Ash's first team (Pikachu, Butterfree, Pidgeot, Bulbasaur, Charizard, and itself) to not evolve or indicate a refusal to evolve.