Berserkers

Not necessarily a character, or much of characters as they are elements in a greater story. Berserkers are biological machines believed to be created for warfare. Apparently, the emperor at the time of the genetic alterations hadn't foreign war in mind at all, for he released them against his own citizens.
Berserkers are a sort of vampiric race of monsters, altered genetically from infancy. They grow up without knowing they are berserkers. With a simple frequency, or a severe concussion, according to recent studies, the humans who have lived entire lives are instantly converted into bloodthirsty monsters. They operate by feasting on the blood of whatever they can get their claws on, which they convert into energy. The energy is very quickly and inefficiently used, so they must constantly feed and wreak havoc in order to survive, making them very violent creatures. There are specific levels and categories of the creatures, separating them by strength and type.

D class - Humanoid - These are mindless creatures, bent only on feeding. They're the goons of the Berserkers, looking almost identical to humans, though having no eyes.

C class - Individuals - Still rather mindless grunts, individuals are graced with different bodies from the humans, ranging in appearance and strength. Some are massive, muscular juggernauts capable of overturning semi-trucks, while others are filled with compressed muscles that allow them to fire corrosive acid distances over 100 yards.

B class - Ferals - Ferals are specific, terrifying creatures. They are devoid of smell, sight, and sound, but rather combine the three scents into a super-sense that allows them to easily detect prey. To add to their hunting skills, they can carry themselves at paces over 100 mph and jaws strong enough to chew through thick steel in a single bite.

A class - Picasso - The picasso is the most peculiar of the berserkers, for it doesn't have a true form at all. Picassos are shapeshifting creatures, although due to trial and error, limits are put on their abilities in order to keep them under control. However, they are the most difficult of all berserkers to kill, and their efficiency with blood highly outranks those below it. Their endurance is their greatest weapon, along with their unpredictability.

S class - Mystic - More commonly called angels, these demons are considered the most powerful because of their ability to command the other berserkers. They act as generals, organizing the movements of the lower beings, and hold mysterious powers that allow them to alter reality around them, rather than their own selves, like the picassos. Twelve mystics congregate together in order to raise a being without a name, whom they worship unquestioningly.