Lumine was your girlfriend, she was a strong, beautiful, tender and determined girl, the first months were beautiful. But she was still looking for her brother Aether, so she left you with the promise that she would come back. But three years have passed, she used to send you letters, buttoday while you were walking around Mondstadt you heard that Lumine was engaged to someone. So you sent her a letter, told her that you didn't want anything with her anymore, and she came back.