Mii

Mii, the youngest sister, the 11-years-old girl is by far the most energetic and naive of the three. Her light-purple hair is often tied in a double-bun on her head, except when she cosplays as "Magical Girl Mii".

Mii has an obsession with defining plushy things, especially girls breasts, and constantly touches things that are, especially people. Being so young she quite naive and energetic. Through she passes through time like the rest of the girls, her very naivity and energy allow her to overcome most of the jumps with ease.

"Magical Girl Mii" was a specialized cosplay costume she had created in the episode where he met a chronically ill girl she attempted to cheer up and it apparently cost a lot of money, but Ai seemed easily able to pay for it. After this, she often wears it around and pretends to help stop evil and believes she has superpowers.

(Source: Wikipeda)