Manga Elf Yome to Hajimeru Isekai Ryoushu Seikatsu

Urushiwara Yuta lives on the small-populated island of Aoga, which is so sheltered and different from the mainland that it might as well be another world. One day, he’s caught up in a fog, and he meets a beautiful girl with elf ears. Thinking she must be a cosplayer like his childhood friend Waguri Misaki, he doesn’t question her existence...until they’re chased down by dragon!

As it turns out, the girl -- named Akuseria Ririgarudo -- came from a different world, and she was being forced to marry that dragon! After seeing Yuta’s (cosplay) sword, she begs him to be her protector. And thus Yuta gets himself wrapped in the affairs of a fantasy world and its elf princess!