Before taking things too far, he eventually catches himself. As the series progresses, Basara's relationship with Mio grows through the multiple conflicts in the series and living their daily lives together. In light novel volume 10, Basara and Mio finally cross the line and have sex.

Did Mio kiss Basara?

Back at their home, Zest pledges her loyalty to Basara. In a moment of passion, Basara reacts to the thought of someone else potentially stealing Mio's virginity. Basara and Mio kiss and just as they are getting closer to having sex Basara stops mid-way, apologizing for forcing himself onto Mio.

Is Basara and Mio blood related?

Basara's biological parents are Jin and Mio+Maria's aunt. As in their father's sister. His surrogate mother is an angel who is the cousin of Chisato. So Mio and Maria ARE Basara's direct blood related first cousins.