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 is quite naive and energetic. Through she passes through time like the rest of the girls, her nativity 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. Later during the episode the girl collapsed from overexertion playing with Mii. Mii tried to use powers she thought she had to heal her friend. The attempt was successful, and Ai revealed to the viewers that Mii's special power was healing. However, Mii remains unaware of this for a good portion of the season, and continues to believe that she has other powers as well.

(Source: Wikipedia)