Jesse (real name: Reverend Horace Black) is the main antagonist of the movie My Babysitter's a Vampire. He was the leader of a destroyed Vampire Cult which he attempts to raise again. He also serves as the main antagonist for the series' first season.
In 1809, Reverend Black founded the town of Black's Church, which would later be named Whitechapel. There, he was the leader of a cult of 219 vampires who terrorized the town.
At some point, a powerful wizard using a deadly weapon, known as the Lucifractor, tried to wipe out all the Vampires in the area. Realising his species was in danger, Black managed to defeat him, by having his followers distract him, while he snuck up behind and sucked the wizard dry. Black took the Lucifractor and hid it away in an old hotel, leaving a shadow beast to guard it.
Eventually sick of his reign, the Humans in the town rebelled, and managed to destroy his followers by burning them at the stake. Black escaped this fate, and swore he would return and take vengeance for what they had done.
In present day, Black returned and posed as a high-school student named Jesse. He planned to resurrect the original members of his cult, as well as amassing new members, including his former girlfriend, Sarah. He was also responsible for her, Erica and Rory becoming vampires (though indirectly on the last one). Ethan Morgan, and Benny Weir, realising the existance of Vampires, team up with Sarah (who refuses to drink blood, thus preventing her becoming a true vampire) to stop Jesse but barely escape with their lives from a party infested by vampires, after trying to save Erica and Rory.
Jesse proceeded to use a relic called the Cubis Animus to steal the souls of people in a movie theater, intending to use them to resurrect his cult members, and retake Whitechapel. Upon Benny and Ethan discovering they themselves were not pure humans (Benny is a wizard, Ethan is psychic,) they, along with Sarah, confront Jesse's henchmen with Anti-Vampire weapons and manage to kill all but him and Erica. Jesse flees the fight and goes to the final resting place of his cult (which coincidentally is Ethan's backyard) to resurrect them. The trio confronted him but being a powerful vampire, Jesse casually held his own against them until Ethan released the souls from the Cubis Animus and as they returned to their bodies, the angry souls turned on Jesse, destroying him.
Following Ethan and Benny being forced to dig up the Cubis Animus to defeat a demon, Jesse returns from the grave. He attempts to persuade Sarah to become a full vampire join him, and tries to force her by using his Vampire powers to enslave Erica. Ethan and Sarah confronts him trying to stop him, but Jesse defeats them and bites Ethan before leaving, well aware of what will occur next. To spare Ethan from becoming a Vampire, Sarah is forced to drain the venom from him, thus drinking his blood, and making her a full Vampire.
After this Jesse was banished by the Vampire Council, who ruled him as to dangerous and being an "Undesirable Element".
Although still banished, Jesse contacts Ethan via psychic link to warn him that someone was looking for the Lucifractor. He explains him what the Lucifractor is, and tells him he had to find it, or else it could used to drive the Vampire species to extinction, his friends included.
In the second episode, following the Vampire Council fleeing Whitechapel, faced with Vice Principal Stern (who is really the descendant of the same wizard who Jesse defeated two hundred years ago, and a vampire-hating madman in his own right,) Ethan somehow contacts Jesse and invites him back to fight with them. Jesse begrudgingly accepted an alliance with his enemies to confront the more dangerous threat. Together with them they faced Stern in the Vampire Council's hall.
Using a variation of the same plan Jesse had used to defeat the orginal wizard, he and Sarah confront him and draw his power, while Ethan snuck up upon on him. Upon catching him, Ethan uses his new power of telepathy to manipulate Stern into trying to absorb the power of the Lucifractor. This causes the orb to explode, apparently killing Stern. It is unknown if Jesse got out of the blast radius in time.