The Queen Leech was one of the most unique monsters to come from the T-virus. Dr. Marcus’ pet leeches began to grow a collective intelligence and came to adore their master. After Marcus’ death the leeches merged with his DNA to create the Queen Leech. The Queen Leech was able to control countless mutant leeches. It also had the amazing ability to become a young version of Marcus complete with all his memories and intelligence. There was no way to tell that the Queen Leech was not human, it essentially became Marcus, its mission was to take revenge on those who betrayed and killed him, Umbrella.

Rebecca and Billy encountered the Queen Leech a few times; in the end the Queen Leech would reveal its true form; a humanoid like monster with large tentacles. One huge advantage of being a mass of leeches was that any damage it would take it could easily replenish itself with more leeches. After one battle with Rebecca and Billy the Queen Leech evolved into a second form, a huge beast like monster.

This new form was almost indestructible; it would absorb all gunshots while unfazed, but it had a huge weakness. The mutated leeches had a transparent coat that allowed UV light to pass through it, but long exposure to it would cause the breakdown of their cellular structure. Rebecca and Billy lured the queen leech into an abandoned factory where they exposed the monster to light thus weakening the beast; Billy was then able to finish the monster off once and for all.