Watashi Wo Yashinatte! Yuusha-Sama

Watashi Wo Yashinatte! Yuusha-Sama summary:

After our protagonist saves a girl dressed as an elf at a doujinshi event, she starts calling him "Hero". He thought it was only a temporary role-playing session, but it seems the girl is serious about it... The curtains open for the difficult days ahead for the amnesiac elf and our protagonist, who was supposed to be an unremarkable person!