Abraham Van Helsing
Chairman of the Hellsing Organization (a.k.a. Abe)
Sold his heart to Rumpelstiltskin in exchange for immortality.
This led to him becoming possessive of Mina Harker and jealous of Dracula, fearing Mina loved Dracula more than him.
This led to a conflict between Dracula and Abraham, where Mina was fatally wounded with a cursed knife.
This led to Mina becoming a vampire, and RSS stealing her away. Hellsing blames Dracula for that, and swore to destroy all the monsters in the world so that no one would suffer as he has suffered.