In Challenge of the Samurai, Squirtle appeared in a flashback, though the episode did not reveal which of the three Trainers who left before Ash owned it. Samurai remembered Gary and the others fondly as they had fought spectacular battles, with Gary's Squirtle defeating his Metapod.
Finally, in The Ties That Bind, the fully-evolved Blastoise was revealed to Ash when Gary sent it out to battle Heracross. In Gary's Full Battle against Ash in Can't Beat the Heat!, its Hydro Pump managed to blast through the rocky terrain and knock out Ash's Heracross in a single hit. It then stalemated his Muk, completely nullifying Muk's Sludge Bomb with its Rapid Spin, but unable to gain much purchase with its Skull Bash. After Muk smothered Blastoise with its body, Blastoise used Hydro Pump to blast Muk off and injure it. Ash pulled Muk for the type-advantaged Bayleef, but Blastoise overcame her as well, deflecting Razor Leaf with Rapid Spin, then using its Bite attack on her leaf and knocking her out by throwing her aside.
Blastoise losing to Ash's Charizard
It was recalled until the final bout when it fought against Charizard, who had overcome Scizor and Golem just as Blastoise had defeated Heracross and Bayleef. With a type advantage and having had a break between battles, Blastoise was quickly on the front foot, deflecting Flamethrower with Rapid Spin and hitting with Hydro Pump and Skull Bash. It neutralized Charizard's aerial advantage by using Hydro Pump to keep it at bay, giving Charizard no opening.
Charizard and Ash thought outside the box and melted all of the rocks with powerful Flamethrowers. Immobilized, Blastoise had no choice but to cool the field down with water, but Charizard took advantage of the resulting steam to get in close to Blastoise. Even then, Blastoise proved a worthy opponent; unable to aim its water cannons it was still able to dodge a Dragon Rage with Withdraw. Charizard lifted Blastoise into the air, holding on despite Blastoise biting hard on its arm, and executed a Seismic Toss. Both Pokémon stared each other down, but Blastoise gave out and fainted.
In Johto Photo Finish! Gary decided to become a Pokémon Researcher and the only Pokémon he would take on his new journey was Blastoise. After using Blastoise to free Pikachu from Team Rocket, he departed home for Pallet and then set off for future endeavors. His choice inspired Ash to begin his journey in Hoenn with only his first Pokémon, Pikachu.
Not much is known about the personality of Gary's Blastoise, however it was described in The Ties that Bind! by Gary that it was his best, showing that it was one of his favourite Pokémon, if not his favourite. It was mentioned in the same episode by Professor Oak to be his most powerful. This was shown in Can't Beat the Heat!, when it defeated Heracross with one move despite Heracross using rocks as a shield as well as it easily beating Bayleef and being a good match for Charizard. Its Rapid Spin can deflect powerful attacks, even super-effective ones such as Razor Leaf.
A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
In I Choose You!
As a Squirtle in I Choose You!
Blastoise made a brief appearance as a Squirtle in a flashback in I Choose You!, which is set in a timeline separate from the main series. Like in the main series, Squirtle was chosen by Gary to be his starter Pokémon.
Unlike in the games, instead of the rival (Gary/Blue) having the starter Pokémon with the type advantage, Ash's Pikachu has the type advantage.
However, Ash uses his Charizard against Blastoise during his battle with Gary in the Silver Conference, mirroring the type advantage seen in the games.
In the English dub, Brock and Misty seemed to be aware that Gary's starter Pokémon was Squirtle, as when Blastoise is shown for the first time, Misty says, "Brock, could that be...?", to which Brock replies, "The same Squirtle that Gary originally received from Professor Oak? Yeah, that's the one." This exchange did not occur in the original Japanese version of the episode.