He is Soubi's former teacher, and held a very influential role in Soubi's life when he was still his teacher. The extent of their master/slave relationship is left unknown, but it is clear that Ritsu misses those days. In the few times they have made contact within the current timeline, Soubi acts very brusque and treats him with disgust. It is also revealed that he was the one who took away Soubi's virginity. Ritsu is said to have been the Sacrifice to Soubi's mother, which led to him and Soubi meeting.
In one of the Zero-sum chapters Ritsu has his eyes gouged out by Seimei, so he could write a message to Ritsuka on the wall in the man's blood.
In the latest chapter of Zero-Sum Monthly Comic (March 2008), the seven members of Septimal Moon were revealed. Included as members were Ritsu Minami, Nagisa Sagan, and Nana. Both Ritsu and Nagisa had previously denied any affiliation with the Septimal Moon.