Jack is a horrible immortal being from Dib's past. He has a strong hatred for humans. He is not an alien but he is a human who is decended from demons. He is European, born in the Celtics, and for some time lived in England before moving to America.
Jack was born in the Victorian era. Because of his Demonic heritage, he has a hatred for all humans, but has a special hatred for women (especially pregnant and nursing women). When he was a young boy, Jack had an encounter with the fallen angel Lucifer who claimed to be a guardian angel. Lucifer provided a knife for Jack to protect himself. Ten years later, Jack had another one, this time Lucifer was more aggressive towards him. Lucifer demanded him to be one of his minions. Jack agreed if the devil could climb a tree. Seeing no harm in this, the devil climbed up a tree and was unable to get down. Jack agreed to help him down if Jack wouldn't be taken to the underworld, to which the Devil was pressured into agreeing with. From that day on Jack was given a talisman made from a single burning coal.
Now that Jack is immortal he dedicates his life to evil. He was known by two names - Springheeled Jack, and more famously, Jack the Ripper. Both personas were known to terrorise London during the 1800s and eluded the police. In the 1970's Jack was a stoner hippie and a nortorious drug dealer known as "Le Noir Loup". He was nearly arrested for numerous drug dealings with the French, Russian and New York mafia, as well as having a role in the death of Jimi Hendrix. In the late 1990's, one of Jack's victims was Alice Membrane, wife of Professer Membrane and mother of Dib and Gaz. A three year old Dib was a witness to this attack, but Jack let Dib live to fear him, a mistake that Jack would one day regret.
Currently Jack is leading a cult that is based in a town outside of Dib's home city. It was that summer Dib, Gaz, Tak, Zim, GIR, and William would eventuly encounter this cult. It is unknown what Jack's plan for the cult was, but he seems to want to contact a mysterious "Johnny C".