Akuninmenshita B-kyuu Boukensha: Shujinkou to Sono Osananajimitachi no Papa ni Naru

In his past life, his favorite game was the RPG Bright Fantasy. Reincarnated as a character in the game world, Gray lived peacefully as an adventurer. One day, he rescued a boy from some street thugs in the city. The boy and his childhood friends turns out to be the protagonist of the game's story. They were living in terrible poverty, and Gray decides to adopt them. That choice will drastically change the fate of this world.

(Source: Kadokawa, translated)