It was mentioned in the festival chapter. After he left to be with his GF, she encourages our MC to chase after a local cat against his instincts (i.e. thinking he wasn't invited by the cat and thus shouldn't reach out even after meeting at the festival) so that he doesn't have any regrets, or something along those lines.
It implied that she may have had feelings for him which were never acted on, and now may never see the light of day given he has a girlfriend already. She serves a guide and facilitating role for our main couple.