Jack Warren was created by Brad Kern for the hit tv-series Charmed. However, all traits in Blessed belong to Matt.
Jack Warren was the son of Viola Thompson and Beauford Warren. Jack was also and the brother of Alice Warren. He never married or had any children.
In Happy Birthday Pip! You're A Witch!, the souls of deceased Warren witches entered Pip Muniz-Halliwell -- who was in his mother's body at the time -- in order for him to overpower the demon, Juno. It's possible that Jack was one of the many Warren souls who helped Pip.