Ash first met Mijumaru in Professor Araragi's laboratory in BW001 as one of the Pokémon Shooti could choose to be his starter Pokémon. He took an interest in Ash after he called him cute, and followed him after he set out on his journey, mostly staying out of sight.
In BW002, Mijumaru rescued Ash's Pikachu and Iris's Kibago from Team Rocket by throwing his shell to pop their balloon, and defeated Jessie's Koromori as well.
In BW003, Mijumaru showed his interest in traveling with Ash and successfully convinced Ash to take him with him on his journey. In his attempt, he knocked Pikachu off Ash's shoulder. When Ash tried to catch him with a Poké Ball, it failed as he was still under Professor Araragi's ownership. Ash immediately called Araragi and requested she send him Mijumaru's Poké Ball. Later in the episode, Mijumaru protects Ash from a hot water spring using Water Gun, but begins to fall into the hot water, which is when Ash uses his Poké Ball to recall him, making him officially his.
In BW004, he was still attempting to get Ash's full attention, to the point where him let himself out of his Poké Ball at the start of a battle, even though Ash had already chosen Pikachu. However, he promptly changed his mind when he saw the opponent was a Futachimaru, his own evolved form.
Ash used Mijumaru in his gym battle at the Sanyou Gym in BW005. He battled against Dent and his Yanappu. He was scared of Yanappu at first, but with some encouragement from Ash and Pikachu, he decided to battle. Mijumaru was petrified with fear when Yanappu started charging SolarBeam. However, as the battle continued in BW006, he was able to use his shell to deflect the SolarBeam, as well as a Bullet Seed attack. He then used Shell Blade to defeat Yanappu, winning the battle and earning Ash the Tri Badge.
In BW007, Mijumaru was used in a battle against a wild Tsutarja. Unfortunately, he was hit by Tsutarja's Attract, and since he is male, he was not able to battle effectively, and was recalled.