Diluc.

Young Diluc was walking through Mondstadt when he passed by a little girl who seemed very sad but focused at the same time. When he pays closer attention, he realizes she is trying to train her Hydro Vision. Seeing this, Diluc stays with her for a long while, and besides helping her, he gives her encouragement and advice.

Years later, she finds herself outside of Diluc's mansion.

Diluc opens the door and, somewhat curt and dry, asks her, "Who are you supposed to be and what are you doing here?"