Squirtle used to be the leader of a gang known as the Squirtle Squad, a group of rogue Squirtle who were deserted by their Trainers. Meowth managed to befriend the Squirtle Squad by convincing them that Jessie and James were his pets. The Squirtle Squad then captured Ash, Misty and Brock so they could take Pikachu back to Meowth. Ash managed to convince the Squirtle Squad to let him free to get a Super Potion for Pikachu, who was badly injured after an earlier encounter with a Goldeen. The Squirtle Squad agreed to let Ash go to town to buy a Super Potion, but if he didn't return by noon the next day, they threatened to dye Misty's hair purple (in the original, they threatened to kill her).
When Ash got back with the Super Potion, Team Rocket began bombing the Squirtle Squad after they received Pikachu. Ash, Misty and Brock ran to hide in the Squirtle Squad's cave; however, when Ash noticed that the leader was stuck on his shell, he went back to save it. Squirtle then defeated Team Rocket with Water Gun. However, Team Rocket's bombs had started a forest fire, which the Squirtle Squad worked together to put out. After putting out the fire, Officer Jenny appointed the Squirtle Squad as the official fire fighters of the town. The leader of the Squirtle Squad preferred to join Ash on his journey rather than fight fires alongside his group, as it wanted to repay Ash for saving his life during the bombing.
Squirtle and the Squirtle Squad
During its time with Ash, Squirtle became one of Ash's three most trustworthy Pokémon, next to Bulbasaur and Pikachu, and it was used very often, about as much as Bulbasaur, Pikachu, and Charmander/Charizard. It served as Pikachu's 'steed' when the group competed in a race, but the two were put out of the running by a Glare attack from Arbok. During the Indigo Plateau Conference, Squirtle won Ash's second round by defeating a Nidorino. One of its most notable battles was when it was pitted against Rudy's Starmie, which was able to use Thunderbolt; despite the power of the electric attack, Squirtle not only defeated Starmie, but learned Hydro Pump in the process.
Ash and Squirtle
During Ash's travels in Johto, it competed in an event called the Fire and Rescue Grand Prix, a firefighting competition for water Pokémon. As luck would have it, one of the teams was the Squirtle Squad, but they were confused without their leader. Ash's Squirtle led the Squirtle Squad to win the Grand Prix, and at the end of the episode Squirtle returned to the Squirtle Squad because Ash felt that the old gang needed Squirtle more than he did.
Squirtle is one of Ash's most trustworthy Pokémon, but is by no means perfect. Compared especially with its closest friend, the very serious Bulbasaur, it comes across as immature and mischievous, particularly during its time with the Squirtle Squad where it led the team to commit various pranks and so on, and how it embarrassed itself trying to intimidate a Snubbull in Pikachu's Vacation. However, through its travels with Ash it matured considerably, and its behaviour when it reunited with its Squad later on had a noticeable difference.
Mature or not, Squirtle is shown to be a born leader and prefers to wear sunglasses occasionally, donning them when investigating Jigglypuff's presence on the island in Beach Blank-Out Blastoise and when it competed against Team Wartortle to prove itself, its fire-fighting spirit hurt by its inability to put out the fire that Team Wartortle had extinguished.
Squirtle is the first of Ash's Pokémon to have been being placed in a Poké Ball off-screen, the other being Lapras.
Though a Squirtle was Ash's first choice before he began his journey, it was the last of the Kanto starters he caught. It should be noted, however, that the order of capture did not depend on him.
Squirtle's gender was hinted at in the Japanese version of Island of the Giant Pokémon. In the subtitles, it refers to itself as おれ ore, which is a pronoun normally used by males, also used by characters such as Ash, Brock and James in the Japanese version.