We Don't all Have to Dance. I can Dance by Myself Episode 11

The mysterious girl is shown to be Serika Gin'yūin, also known as Sericchan, the idol loved by Japan. She is supposed to held a concert during the final day of the festival. Souta and Nanami learnt that Sericchan managed to become an idol because Souta saved her, by saying she has a "Success Flag", though she doesn't know the identity of that person, and she named him as "Purple Flagman". Souta realized that Sericchan is the Bard. During the concert, Souta's power increases once more, and the time freezes up. No. 0 appears and tells Souta that he should not be in the world, or the world will be destroyed. So, despite Souta had promised the Quest Dorm members to dance with them, he breaks the promises and uses his power to make everyone forgets about him. Nanami, unaffected by Souta's power, is the only person who is able to remember Souta.