One of the strangest creatures to come from the T-virus, the Mimicry Marcus is actually countless leeches bunched together to form the image of their master James Marcus. The leeches bind together and secrete an adhesive bodily fluid to mold together; they seem to come together because they share a collective consciousness.

The Mimicry Marcus begins to lose its shape when attacked; however, it becomes highly aggressive. It will attack its enemies with its hands, extending them and attacking in a whip like fashion. They are very difficult to kill as Rebecca and Billy found out while escaping the training facility. They found that the only way to stop them was to use sulfur or burn it. When the Mimicry Marcus is critically damaged it will explode, scattering leeches all around the area.