I'm looking for a manga where the MC returns after defeating the demon king and goes to the country side to retire. He starts living a slow life there and meets a girl and makes friends with her. This girl already knows him as she was rescued by him when she was young and fell in love with him. She immediately recognises him but he doesn't recognise her. But they slowly get to know each other and they find out they knew each other as well.
This is likely A-Rank Boukensha no Slow Life.
Aldred, the leader of a kingdom’s A-ranked party, "Black Silver", has accomplished the task of slaying a dragon. However, he who has pursued strength his whole life has lost sight of his goal from taking down a monster of the strongest species. As he worried about what to do next in his life, it was the scenery of Nordende that popped up in his mind; A rural village by the countryside, full of natural beauty, which he only visited once during a quest in the past. After declaring the disbandment of his party, he heads towards Nordende for a comfortable, relaxing life.
There are some differences, namely the main character and his party defeat a giant dragon, not the demon lord. The basic idea is the same - he retires after defeating a particularly strong foe.
When he arrives at the village he meets the chief's daughter, a young woman, and it is eventually revealed that he saved her years ago. She probably recognizes him immediately, but this is only explicitly shown to the reader after several months (and 7 chapters).