Read The Great Sage Who Returned From Another World Wants To Live Quietly manga
Saito was reincarnated to the other world at the age of 15, becomes the Great Sage, and defeated the Demon Lord. Shortly after that, the door connecting the other world and the real world opens, and Saitou return to Japan with the power of the Great Sage. However, 19 years have passed in the real world. Saito, who was drifted away for so long, met his childhood friend Kanako, who’s now a single mother. But it seems like there’s someone who’s targeting Kanako’s life… A story of a slow life wannabe Great Sage who came back from another world begins!