Doru

What we know

Son of Father Donavich; vampire spawn trapped in church undercroft (Session 4 @ 01:18:43)
Led revolt against Strahd von Zarovich with priest's son Oru; returned transformed (Session 4 @ 01:18:43)
Betrothed to missing Gertruda (Session 4 @ 01:48:31)
Passed one-minute fresh-blood resistance test; fangs retracted; party spared him (Session 4 @ 01:45:41)
Returned father's holy symbol to Mercurio (Session 4 @ 01:40:23)
Shared vampire spawn abilities and weaknesses with party (Session 4 @ 01:57:34)
Named Dr. Rudolph van Richten as famed hunter who failed against Strahd (Session 4 @ 02:02:58)
Once owned a copy of van Richten's Monster Hunting Guide; copies said to be circulating in Barovia (Session 6 @ 00:59:22)
Saturno left notebook; party promised pen and sheep blood (Session 4 @ 02:09:53)

Church undercroft during Session 4 (Session 4 @ 01:30:23).

Chanted Morning Lord prayer during blood test; whispered "Gertruda" (Session 4 @ 01:44:09)
Sildir remained hostile with arrow drawn throughout (Session 4 @ 01:41:25)