Kanto Battle Frontier
After leaving her Beautifly and her Skitty in Hoenn, she met up with Ash in Pallet Town, where one of Professor Oak's Squirtle took a liking to her. When it did not want to go in Professor Oak's Poké Ball, he allowed it to travel with May, so it went in her Poké Ball instead.
May quickly introduced it to Pokémon Contests, where it adapted well. It was first used in the battle rounds of the Saffron City Pokémon Contest, defeating Harley's Ariados, which was its first major accomplishment on May's team. It also managed to defeat Drew's Flygon in the Kanto Grand Festival with the help of May's Combusken, yet another of its many feats.
Squirtle was used for the Wisteria town contest in Harley Rides Again where it gave a great preformance allowing May to compete in the battle rounds.
Sinnoh
It was assumed that this Pokémon continued to remain on May's team, traveling with her through Johto. Squirtle evolved in time to enter the Wallace Cup with May in Staging a Heroes' Welcome! May used Wartortle in the appeals round.