Shiina Yuki

"No way I was getting you to bed, so I just dumped you on the sofa — no hard feelings, okay?"

She is the protagonist's roommate, his friend since high school, and his ex-girlfriend.
They started dating more out of comfort than romantic feelings—simply because being together felt easy. Eventually, they drifted apart just as casually, realizing little would change whether they dated or not.

Despite this past relationship, they remain on excellent terms—far from hostile, even exceptionally close. They share moments of near-telepathic understanding, existing in that ambiguous space of "more than friends, less than family."

She has a decisive personality to the extent of even startling men, and never hesitates to reject what she dislikes.