Responses
I think instead of Sugi, it was just his husband and then the 5th would be Umesaki (the hot mommy with the short hair) since she seemed to know Utsugi as his highness (in the investigation scene of the pairing party murder)
Ok wait I just reread it again and counted the people in the last panel of vl 3. So it's his highness and 5 knights = 6 people, so I think you're right in that's its Sugi & his husband! I also still think Ume is also an origin vampire. Or it could unexpectedly be the cowardly vampire (who was too scared to see his ex at her wedding)
So the ancient vamp is 5 ppl. The emperor is Utsugi, Aogi the Captain, Megi was the knight. The 4th one I think it's Sugi. And the 5th maybe Sugis deceased husb.