Mui (Japanese version: Ai (アイ, Ai?) is Makoto's older sister. She accompanied Takashi and Kosuke during the battle in episode 9 of Bakugan Battle Brawlers. She is characterized by her unwillingness to see Bakugan sent to the Doom Dimension.
Mui has mid-length blonde hair, a small segment of which was braided, and blue eyes. She also wore a pink shirt.
Her first appearance was in Fight or Flight, accompanying Takashi and Kosuke, stating that she would always wait for Makoto there. Later, she followed her friends, who wanted to reunite her with her younger brother, Makoto, by defeating the Brawlers for Masquerade into the battle with Runo and Marucho. Despite seemingly being on the winning side, she was pained to see Bakugan being sent to the Doom Dimension.
It is later revealed that she and her brother Makoto were separated because of their parents allegedly divorcing. Watching the Bakugan get sent to the Doom Dimension reminded her of her good memories with her younger brother and friends, playing Bakugan so happily, so she could not bear watch it.
After bursting out crying and begging the boys to stop, she watched them finish the battle without the Doom Card, and gradually became happier. At the end, she gave each of them a kiss as an expression of gratitude.
In the last episode of the season, Mui was seen with Makoto and her friends briefly, implying that she has reunited with her brother.