MTL - Mermaid Effect - Chapter 166

An Individual from Earth finds themselves in the Shinobi World, equipped with a Scientific System. From that moment, he delved into scientific research far ahead of its time.

When I cleared the final chapter of a game that was a national failure, I became an unnamed side character in it.

A non-standard, unmeasurable, EX rank supporting character.

This is an extremely difficult game, if you don’t have enough skills, don’t even think about going out!

- Description from Novelupdates

The story is about a boy entering high school, who answers a strange call from a public phone one night. The caller challenges him to a bet: She’ll remove the birthmark which he sees as the cause of so many problems, and he’ll have fifty days to win the heart of a girl he’d considered far beyond his reach. And with this, a very strange summer begins.