May's Wartortle was received from Professor Oak while it was a Squirtle.
It debuted in The Right Place at the Right Mime. Squirtle became attached to May while she was visiting Professor Oak's lab and decided to travel with her. It instantly took a liking to contests and learned Ice Beam without any training in order to defeat Team Rocket. Seeing Squirtle's potential in contests and in battle, May used it many times in Kanto, including her battle with Drew in the Kanto Grand Festival alongside Combusken against Drew's Flygon and Absol. After some tough battling, Squirtle dodges Absol's Flash and Flygon's Steel Wing with Rapid Spin, and strikes Flygon with Bubble. Squirtle knocks it out with Ice Beam, but the celebration is cut short when Absol harshly slams Squirtle into the arena wall with Iron Tail, which left Combusken and Absol to battle 1-on-1 for the second half of the battle.
At some point before the Wallace Cup, Squirtle evolved into Wartortle and learned Aqua Tail. Wartortle does a phenominal job in the appael round and gets May to the next round.