long story short.
Mc was traveling and heard a rumor of a monster in the forest. And he wanted to check if he can defeat it.
The monster known as avenger. A bearlike monster with a mouth of a worm full of spikes. Notorious for hunting anything that harms it till death.
Mc escape form it in a storm by drowning in a river and returning to Japan. He drown to preserve his equipment.
He woke up on a branch. And fell on a bathing Marie. Marie in panic summons undone are rip mc in to shreds.
The river was colored red for a moment, but return to normal the next.
Being left alone Marie who just killed a boy, was shaken. She also could not understand what she just saw. And also did not know human language at the time.
In fact she was a human hater(not that she knew one)
Mc who heard Marie casting magic, liked how the elven language sounds decided to learn it. But he had to restart as everything he had was shredded by Marie's attack.
Next time they meet Marie thinks he is a ghost that is haunting her. Mc thinks there is a monster near by and followed her while on guard (while wearing leaf skirt and a stone axe)