Jessica Priest is a fictional character from the Spawnuniverse. Specifically, Priest serves as a substitute in the Spawn film for Chapel, Al Simmons's killer in the comic book. Priest was played by Melinda Clarke in the film.
The character of Chapel (originally from the comic book series Youngblood) could not be used because the rights of the character were owned by Rob Liefeld. McFarlane created a new character to replace Chapel in the movie, and later had Priest retconned to be Spawn's killer in the comic as well. Some of her background is revealed in Curse of the Spawn #12-14.