A cheerful girl who seems like the polar opposite of the quiet Mio.

Spoiler

Mio's imaginary friend who appeared when Mio was bored in her younger days. She was forced from Mio's body by therapists, which left Mio with half a shadow. She has the other half. She reappears as Mio is being initiated into the Little Busters, which causes Mio to disappear as a seagull and Midori to take her place as Mio. Even though nobody else notices a difference, Riki goes to save Mio. In the end, Midori becomes one with Mio.

Spoiler

In Mio's Bad End, everybody sees her as the original Mio Nishizono and while Riki tries to resist at first, he gives up and sees her as the "real" Mio too.