A Goodreads user asked:
I'm not sure what Silas is? Tall, pale faced, cold, not alive, yet not dead... is he suppose to be a vampire?
Jitesh Banga Another prevailing theory is that Silas is the Son of Dracula. He mentions that he was a monster at the start, but now no longer the monster he once was. Also, with the way the werewolf, Mrs Lupescu followed him naturally, as wolves followed Dracula, also hints toward it.
Debi Hawkes The author clarified all the hints toward the end when he mentioned Soles did not have a reflection.
Annak I think he is a vampire, yes, but I like that 'what' he is is open to discovery and interpretation.