The Queen Leech (女王ヒル,joō-hiru?) was a mutant leech created through experimentation on annelids with the t-Virus. Created as part of Dr. James Marcus' ongoing leech study, it was his final creation before his death. In the time after, it consumed his hippocampus and gained his memories. Believing itself subsequently to be Marcus reborn, it sought revenge for his death.
The Queen Leech was created in 1988 in a laboratory adjacent to Umbrella's executive training school in the Arklay Mountains. When Dr. Marcus was assassinated by the Umbrella Security Service, Queen Leech was dumped in a nearby treatment plant with his corpse. Seeing his body as a food source, Queen Leech began consuming it, and gained his memories after consuming the brain. It believed itself to be Marcus himself, reborn by divine intervention.
Over the next ten years, the Leech developed the ability to change its body shape into that of Marcus' . In May 1998, the Queen Leech finally acted on its plan to avenge Marcus' death. Over the next few weeks, it manipulated an accident at the Arklay Laboratory, leading to the entire facility staff being infected with the ε strain, while their B.O.W.s escaped into the forest. Umbrella's investigation teams were sent to evaluate the training school and laboratory for recommissioning were attacked and infected by leeches, as were the entire complement of a passenger train transporting Umbrella staff through the Arklay Mountains. The Queen Leech's goal was to create a devastating viral outbreak that would ultimately spread across the world. In keeping with Marcus' Christianity, Queen Leech saw itself as Jesus Christ and judged the entire planet to be wicked and be condemned to Hell.
These attacks got the attention of Drs. Albert Wesker and William Birkin, who were in charge of the training school operation. The two quickly recognized the Queen Leech as a serious threat to the company and made preparations to destroy the facility. In the meantime, the Queen Leech's attention was focused on S.T.A.R.S. member Rebecca Chambers and former Marine Second Lieutenant Billy Coen, who found their way in the training school when the train crashed into the underground station beneath it. The Queen Leech eventually saw no other way than directly confronting the two after they survived repeated attacks from Marcus' B.O.W.s. It abandoned its human form and transformed into its natural state, a large green monster. The fighting went awry when it accidentally revealed to Billy its sensitivity to sunlight. Distracted by the opening of a shutter, Billy fired at the Queen Leech with a high power magnum, finally killing it and its broken body fell down an elevator shaft where it was consumed by flames, as Dr. Birkin set off the self-destruct during the fight.
The Queen Leech's transformation in its last moments was eventually found out by Wesker some time after the events at the Umbrella Executive Training School, which led to his theorization of a connection between Progenitor Virus mutations and the psychology of the infected.