Akuninzura shita B-kyuu Boukensha: Shujinkou to Sono Osananajimi-tachi no Papa ni Naru

Gray, a B-rank adventurer, was supposed to be just a mob character in the RPG "Bright Fantasy." However, by chance, he saved an orphan who was attacked by thugs on the roadside and decided to raise him as his father. But the orphan turned out to be the hero of this world, "Iska." This choice would greatly change the fate of the world.

(Source: Kadokawa, translated)