Saitei Rank no Boukensha, Yuusha Shoujo wo Sodateru Orette

The old man who joined just to fill out the party is actually a legendary adventurer!? A hidden hero turns a talented S-class party of beautiful girls into full-fledged heroes! The greatest training fantasy from "Let's Become a Novelist"!! Meet Kousuke Igami, an unassuming middle-aged man, commonly seen as nothing more than a placeholder in his mundane life. But beneath this ordinary facade hides an unexpected reality: he is, in fact, a legendary adventurer! As destiny would have it, he finds himself mentoring a group of prodigiously gifted and enchanting young women.