The Queen Leech was a singular Progenitor- infected leech that appears in Resident Evil Zero. James Marcus, co-founder of Umbrella and the man who developed the T-virus, was conducting tests on this specimen at the time of his assassination. When Ozwell Spencer's agents gunned the doctor down in his lab, the container that held the Queen - which still looked like the other infected leeches at the time - was shattered, and the creature fell from the table and dropped on to Marcus' dead body, crawling inside his mouth. While Marcus' body was discarded in an old water treatment plant, the virus within the leech allowed it to merge with Marcus' DNA. Melding with Marcus' body, the leech gained a new physical form and even assimilated Marcus' memories. This process of biological fusion took ten years to come about and Marcus was essentially resurrected early in the summer of 1998.
Assuming the form of James Marcus from his youth, the Queen Leech - having gained a measure of sentience through Marcus' memories - became motivated by revenge against the Umbrella Corporation and sought to destroy the organisation. Able to control the infected leeches through thought, the new Marcus attacked the Arklay Mansion facility and scattered the T-virus, infecting the lab's entire staff. He also attacked the Ecliptic Express train, infecting all of the passengers and turning them into zombies. Marcus effortlessly shirked off U.S.S. commando attempts to clean up the source of the biohazard. In control of the situation, Marcus displayed a video transmission across the training facility, where Albert Wesker and William Birkin remained in hiding from the zombie population, threatening his murderers that they would pay for their act of betrayal.
Marcus' plans would be undermined by the interference of Billy Coen - an ex-Marine - and Rebecca Chambers of the Special Tactics and Rescue Service. The pair survived the derailing of the Ecliptic Express, fought their way through numerous zombies and B.O.W.'s within the Umbrella Research Center and came face-to-face with Marcus inside the treatment plant. It was then that Marcus mutated into a much less human form, revealing its true monstrous nature. Rebecca and Billy fought valiantly against the Queen Leech as they tried to escape from the facility, but it was only after luring the Queen into sunlight that they discovered its weakness and defeated it.
- Shapeshifting - The Queen Leech had four different forms: one as the elderly Dr. Marcus, the second as Marcus' younger self, a humanoid leech form and a massive, bestial form.
- Invulnerability - While not completely invulnerable, the Queen Leech was exceedingly resistant to attack, seemingly absorbing bullets like a sponge soaks up water.
- Tentacles - In its first mutant form, the Queen attacks using several tentacles to whip Billy and Rebecca.
- Acid - In both mutant forms, the Queen could spit globules of toxic acid at enemies.
- Leech Control - The Queen Leech can control other leeches, commanding them to attack prey and even fuse together to form 'Mimicry Marcuses'.
The Queen Leech's only real weakness was sunlight. After Billy opened a large blast door leading outside, the Queen was burned by the sun's ultraviolet rays and its thick, spongy tissues became much more fragile, allowing Billy to kill the creature for good with a well-aimed Magnum shot.