Mijumaru originally belonged to Professor Araragi, as one of the three starter Pokémon a new Pokémon Trainer could choose. Ash called him cute when he was sent out in BW001, when Shooti had to decide between the three starters. He then became extremely shocked when Shooti chose Tsutarja. Soon after, he started taking interest in Ash, observing him but staying out of sight, and to the point that he left Professor Araragi's lab to follow him.
Eventually, Mijumaru expressed interest in traveling with Ash and convinced Ash to take him along on his journey in BW003. Ash only realized that it was Professor Araragi's Mijumaru when he failed to capture it in a new Poké Ball, and had to obtain a worried Professor Araragi's approval to allow Mijumaru to travel with him and to request for Mijumaru's Poké Ball.
Mijumaru, like some of Ash's previous Pokémon, had constantly sought attention from Ash. In BW003 when he wanted to travel with Ash, he jealously knocked Pikachu off Ash's shoulders and pretended that he was as close as Ash was to Pikachu. Then, when Ash was at the Pokémon Battle Club in BW004 battling another trainer, he came out of its Poké Ball, even though Ash had already clearly chosen Pikachu. However, he promptly changed his mind to battle when he realized that his opponent was Futachimaru, a Pokémon of his own evolved form.
Mijumaru has a sea-shell shaped appendage on its belly that can be detached and used to battle, and occasionally throwing, as it did in BW002 when he popped Team Rocket's balloon, thereby saving Pikachu and Iris's Kibago from captivity. Mijumaru's shell made is made of keratin, and being light but strong, it is capable of deflecting attacks, prominently shown when he deflected Yanappu's Bullet Seed and ultimately SolarBeam in BW006. During that decisive match for the Tri Badge, he subsequently used its shell in a Shell Blade attack to defeat Dent's Yanappu, earning Ash a victory.
Mijumaru was revealed to be male when it battled a wild Tsutarja that would be caught by Ash in BW007. He became infatuated when hit by the Grass Pokémon's Attract, and was eventually recalled.
In BW010, it was used to fight against Shooti's Hatooboo. He was defeated by a single Aerial Ace.