The Old Wizard made his first appearance while Simon Belmont was visiting his ancestor Trevor's grave. He said that he had known Trevor Belmont and even helped him defeat the Count a hundred years ago, which made Kevin question the truth of the (fake) Poltergeist King's story.
The Old Wizard accompanied Kevin and Simon on their search for the (real) Poltergeist King, but accidentally teleported himself away while trying to use a spell to save them from a trap. He made another appearance at the second award ceremony, where he used a spell to end Simon's drawn-out acceptance speech.
The Old Wizard is probably based on Sypha Belnades, one of the protagonists of Castlevania III: Dracula's Curse. If Sypha remains Trevor's companion up through the end of the game, however, he's revealed to be a young woman, and later on marries Trevor Belmont.