cmiiw. Yuri is in love (or is it just a possessiveness, idk) with his oldest noona. they're not blood related it seems, the noona is adopted. and the teacher (Sunwoo) is in love with noona's husband. Yuri is pissed with the teacher because he thought that Sunwoo also loves noona.
oh hell no.. if it was sahn spreading all those rumors about dojin