Nana to Kaoru (2011)

Kaoru is a 17-year-old virgin who has an S&M fetish. He has always dreamt about an SM relationship with his childhood friend Nana, who is looked upon as perfect and is always the target of boys' eyes. Their relationship has somewhat deteriorated through the years, due to them hanging out with different people and because Kaoru is not taking his education too seriously. One day though, Kaoru's mother gives some of his S&M toys to Nana for safekeeping - Nana finds the leather one-piece that Kaoru bought and tries it on but accidentally locks herself in it and doesn't have the key! Kaoru frees her of the one-piece, but then Nana realizes she may like Kaoru's hobby more than she thought and subsequently the two of them start engaging in so-called "breathers". These breathers are intended to relieve Nana of stress, stress originating from her attempts at keeping her grades and her name perfect.