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There is a saying: "When a sub can make his/her dom do what he/she wants, than it opens the question who the real sub is."
So basicly, dunno lol.
In the end the main question is not whatever Nana is really into being a sub, but more if it's actualy a way for her to express her feelings for Kaoru by putting so much trust into him.
Also it's a manga for guys, showing tied up boys wouldn't make much sense.
