During the Kijuju Incident, a number of abnormal insectoid creatures appeared within TRICELL's Uroboros Laboratory. These creatures were not B.O.W.'s as they were created entirely by accident after common cockroaches inadvertantly became infected with the Uroboros virus.
The virus caused the insects to grow to surpass the height of an average adult human male and the two front pairs of limbs extend into rigid, razor-sharp blades; the protective chitin covering the body is divided into flexible plates that can easily shift, allowing a Reaper's muscle tissue to expel a strange gas from the gaps in the carapace. This gaseous expulson has a kind of 'mirage' effect, blurring the Reaper's immediate surroundings to the naked eye and making it difficult for someone to aim a weapon at the creature's vital areas.
As well as being impervious to pain and lacking any sense of self-preservation, Reapers are also capable of quickly regenerating any wounds. Any chitin, limbs, even the head will all grow back in mere seconds if severed.
Reapers are utterly violent and impossible to control. When Albert Wesker first discovered one of these monsters, he sent an entire squad of Majini soldiers to capture it. The beast managed to slaughter the whole squad, disemboweling them one by one. When Chris Redfield and Sheva Alomar encountered these creatures, they had considerable difficulty in dispatching them. Reapers are mindless and do not distinguish between friend and foe, relentlessly pursuing any living creature they encounter regardless of risk to themselves.
While Reapers are both resilient and capable of rapid healing, they are not indestructible. When a Reaper sustains injury to its abdomen or thorax area, a bright swelling will appear from the breach in the creature's carapace. Gunfire to this swelling area will cause fatal shock to the creature. Automatic pistols, shotguns and machine guns should suffice in causing fatal damage. Also, heavy explosives such as rocket launchers are useful as they will completely obliterate a Reaper without the need for precision.