mouto wa Kanojo ni Deki Nai no ni is a Manga/Manhwa/Manhua in (English/Raw) language, ACTION series, english chapters have been translated and you can read them here.
All of the strongest have been raised by the “Mentor” A gentle young man named Haru is living in a run-down church in the outskirts of a frontier city, but his real identity is the legendary “mentor” whose pupils’ names are famous throughout the entire continent. The next target of his guidance is Rebecca, a mid-rank adventurer whose progress had plateaued out. How will Haru nurture the latent abilities of a girl who has given up on herself!?