Ivor is a vampire. He sired Tobias Mathews and left him the following night to fend for himself. After Tobias' return in Heart and Soul Ivor concocts a convoluted plan to enlist Tobias and Dorian in eliminating his greatest enemy. After reuniting with Tobias he convinces Tobias and Dorian to meet him at a circus. Once at the circus Dorian appears to be intoxicated, he is giddy and playful immediately after having a bitter confrontation with Tobias. Ivor later explains that Dorian was being influenced by an ancient siren.
The Siren has been bound to Ivor for over five hundred years. She exerts an intoxicating influence over the humans attending the circus improving their moods and opening their wallets. Her only price is one still beating human heart per night. For centuries Ivor has been her hatchet man. He wants out and he needs Dorian to do it.
Dorian reluctantly agrees to serve as bait for the siren to give Tobias and Ivor a chance to kill her. Dorian is served up only to realize that he has been set up. The siren devours his living heart while Ivor beats Tobias unconscious when he tries to save Dorian.
When Tobias recovers Ivor explains that the siren was what had attacked Tobias centuries before not a bear, that the attack was the reason Ivor had to sire Tobias as a vampire. He engineered Dorian's sacrifice to ensure the siren would be well fed with Dorian's regenerating heart, thus freeing Ivor to lead a life with Tobias. Incensed and disgusted Tobias tries to escape only to be prevented by Ivor. Tobias attacks Ivor and frees Dorian, they then trap the siren in an oil drum and send her to the bottom of a lake.
While not confirmed it is very strongly implied that Tobias killed Ivor.