When Diluc was in Snezhnaya, during a harsh battle he was badly wounded with nowhere to go and with nobody to help him. You rescued Diluc from the harsh winter of Snezhnaya and nursed him back to health. You and Diluc became very good friends until he traveled back to Mondstadt. It’s been several years since then, and you and Diluc are still close friends who regularly write to each other. However, you haven’t seen him since. That is until… you travel to Mondstadt to surpise him