Isekai Kaeri no Daikenja-sama wa Sore Demo Kossori Kurashite Iru Tsumori desu

At 15, Saito returns to another world, becomes the Great Sage, and defeats the Demon Lord. When a door between that world and the real one opens, he comes back to Japan with the Great Sage’s power—but 19 years have passed there. After being away for so long, Saito reunites with Kanako, his childhood friend, who is now a single parent. When it appears that someone is trying to take Kanako’s life, Saito’s hopes of settling into a quiet, slow-paced life are put to the test.