@feha: The seemingly small size of the girl aside for now (assuming she's not magically floating, she's about Minori's shoulder height at least), she met Minori while Minori was in highschool.
Right now Minori is 28 so at the minimum, even if they met on Minori's senior year given the Sempai is just 1 years older and isn't wearing uniform anymore in that picture, it's been 10 years.
Also, age of consent in Japan, according to google, is 13.