I think it's love (to Yumi for Sachiko, Rei/Yoshino, Shiori/Sei, Canina for Sei...) but how to define it ?
there is no precise act, but all is in the heart
it is necessary to acknowledge that their relations are too ambiguous to be friendship. especially when Sei said "Sachiko, do not worry, we won't take off our clothes to play with our bodies! " i loved the face of Sachiko at this moment !!! then, it's not friendship...