Blastoise debuted as a Squirtle in Summer of Discovery!, where he was first seen battling a Trainer's Conkeldurr. He easily managed to outmaneuver the Muscular Pokémon with his dance techniques and defeated it with Skull Bash. Squirtle later participated in a first-touch battle against Ash's Froakie and won.
In Day Three Blockbusters!, Squirtle starred in a Pokévision video along with his Trainer, Tierno, Shauna, and their Pokémon.
In Battling Into the Hall of Fame!, Tierno used Squirtle in a Team Battle against Ash's Pikachu, Serena's Fennekin, and Clemont's Chespin alongside Shauna's Bulbasaur and Trevor's Charmander. During the battle, he managed to defeat Fennekin with a super-effective Aqua Tail. He was the last Pokémon of Team Squirtle to get defeated when he was hit by Pikachu's Thunderbolt.
Squirtle appeared in a flashback in Showdown at the Shalour Gym!. He was also seen battling Clembot and his Heliolisk at the Lumiose Gym in a flashback. While the outcome of this battle is unknown, Tierno did win the Voltage Badge.
In Good Friends, Great Training!, Squirtle was revealed to have evolved into Wartortle. He first performed a dance with Tierno's other Pokémon, but was later captured by Team Rocket along with Raichu and Ash's Sliggoo. After Team Rocket's plan failed, the Pokémon reunited with their Trainers and Sliggoo evolved into Goodra. Later, Wartortle was used in a Tag Battle alongside Raichu against Pikachu and Goodra. He fought well but lost.
In Tag Team Battle Inspiration!, Wartortle was revealed to have evolved into a Blastoise. He participated in a Tag Battle along with Ash's Pikachu against Shauna's Ivysaur and Serena's Eevee, in an attempt to get the latter Pokémon to overcome its shyness. Blastoise's dancing inspired Eevee to respond with her own dancing, allowing her to evade Blastoise's attacks and land a hit with Swift until Serena called the battle off in order to practice for her upcoming Pokémon Showcase.
In Valuable Experience for All!, Blastoise was used during Tierno's battle against Sawyer in the quarterfinals of the Lumiose Conference. He was sent out after Tierno returned Raichu to his Poké Ball due to his infatuation with Slurpuff, whose Attract had no effect on Blastoise. As a result, Blastoise was able to hit the Meringue Pokémon with a powerful Hydro Cannon, rendering him unable to battle.
After that, Blastoise went up against Sawyer's Sceptile. He managed to stand his ground for a while, even resisting a super effective Leaf Blade, but ended up losing after he got hit by Frenzy Plant. Before he fainted, he used Rain Dance to change the weather and make things easier for his teammate Raichu.
In A Towering Takeover!, Blastoise joined the fight against Team Flare. He was later seen watching the battle against the Giant Rock unfold on television in Rocking Kalos Defenses! and Forming a More Perfect Union!.
Blastoise is Tierno's starter Pokémon, which Tierno received from Professor Sycamore when he was still a Squirtle.
He is a friendly Pokémon that often dances with his Trainer, but can be annoyed if attacked, as shown when Raichu accidentally hit him with Thunderbolt. As a Squirtle, he used his nimble dance moves to outmaneuver his opponents in battle, allowing him to defeat much larger and imposing Pokémon such as Conkeldurr, and giving him an upper hand over Ash's Froakie. He maintained his agility even after evolving into the much more bulky Blastoise.
Blastoise is very loyal to Tierno and shares some personality traits with his Trainer. He is a skillful dancer and he is able to easily mimic Tierno's dance moves.