I know there's a theory that the missing 10th Harbinger is Diluc's father Crepus. But I think it was Crucabena. When Arlecchino killed her, the Tsaritsa would have given her the 10th seat. But upon finding out about her Khaenri'ahn curse, she instead gave her the 4th seat.
Now the question changes to "who was in the 4th seat before The Knave?" And to that, I have no answer lol #GenshinImpact
