Name: MARCIE ROSS aka INVISIBLE GIRL 
Status: Human
Traits/Powers: Invisible and very shy
First Appearance: "Invisible Girl" 
Show: Buffy The Vampire Slayer
Other Info: Giles revealed that Marcie Ross was in everyone's classes, but no one seems to even remember that she existed. Giles says she simply became invisible because she was perceived that way -- it's all due to physics, helped along by living above a Hellmouth

Name: JACK O'TOOLE
Status: Zombie, Deceased
Traits/Powers: None
First Appearance: "The Zeppo"
Show: Buffy The Vampire Slayer.
Other Info: Tried to blow up the school but is thwarted by Xander and only Xander.

Name: JACK
Status: Human
Traits/Powers: Family member is a witch.
First Appearance: "Beer Bad"
Show: Buffy The Vampire Slayer
Other Info: was sick and tired of the students that thought they were better than him so he used a Warlock's recipe to make some Beer to change them into Cro-magnon men. 

Name: JESSE (bottom right)
Status: Vampire, Deceased
Traits/Powers: None
First Appearance: "Welcome To The Hellmouth"
Show: Buffy The Vampire Slayer
Other Info: He was Xander's best friend and then he was turned into a vampire.

Name: THE JUDGE
Status: Demon, Deceased
Traits/Powers: Can Incinerate with a thought
First Appearance: "Surprise"
Show: Buffy The Vampire Slayer
Other Info: An ancient and indestructible demon warrior, the judge once roamed the earth, laying waste to human civilization in the name of all that's evil. Since it was not possible to kill him with normal weapons, the only way to defeat him was to chop him up and separate the pieces from each other. As a birthday gift to Drusilla, Spike orchestrated the Judge's resurrection, bringing the pieces back together. Once his parts were joined, the Judge was reanimated.

Name: JUNGLE BOB
Status: Human, Deceased
Traits/Powers: None
First Appearance: "Homecoming"
Show: Buffy The Vampire Slayer
Other Info: Also a participant in "Slayerfest 98"