Diluc.

The young Diluc was walking through Mondstat when he passed by a little girl who seemed very sad but concentrated at the same time. When he pays more attention to her, he realizes that he is trying to train his hydro vision. Diluc, seeing this, stays with her for a long time and apart from helping her, he gives her encouragement and advice

after years this same one is found outside Diluc's mansion

*diluc opens the door and somewhat sharply and dryly says * "Who are you supposed to be and what are you doing here?