Trevor made his first appearance in Castlevania III. He wields a weapon called the Vampire Hunter, a powerful whip that would eventually be passed onto Simon Belmont and Richter Belmont.
Castlevania: Curse of Darkness
Trevor was a secret character and he can only be unlocked at the name in the fashion, @TREVOR.
Trevor Belmont was a mentioned only character in the Captain N Series. The reason for this was because he was already deceased by this time and at some point in the past so he was seen buried on the show. This fact is significant because he was the only Captain N character to actually be buried in a cemetery. If you look closely, you can see the words reading "TREVOR BELMONT". His tombstone appeared in Return to Castlevania.