When Seta makes a surprise visit to the Hinata House, Mitsune attempts to prevent Naru from seeing him and rekindling her crush for Keitaro's sake.
Enlisting the help of the other dormitory residents, she barely manages to keep the two from meeting, however, Naru runs into Seta later during the applications for the university exams and, after overhearing Keitaro offering Naru his help in her confessing her crush, Mitsune also offers Naru her support; however, their efforts are completely wasted on the clueless Seta.
As Naru and Keitaro's relationship waxes and wanes, Mitsune continued to offer her support, frequently pressing Naru on her trues feelings for Keitaro despite the girl's stubborn denial.
However, Mitsune soon became frustrated with the couple and attempted to further the relationship by knocking Naru unconscious and enticing Keitaro by mimicking her.
A bit of a tomboy in her school years, Mitsune and Naru Narusegawa have been friends since back in grade school and it was by her invitation that Naru moved into the Hinata House.