A space bacteria that gets inside the human body and feasts on the host’s fibrinogen a protein necessary for blood-clotting. Infected humans rapidly lose platelets in their blood and turn into a vampire, unconsciously wandering around in a sleepwalk-like state while seeking for blood.

Darii hid in its egg that resembles a beautiful flower petal and infiltrated a University student called Kaori when she kissed what she thought was a flower petal. Darii infected her lungs and over time it turned her into a vampire.

Kaori wandered around in search of human blood so Ultraseven shrunk down to micro size and entered her body. Darii fought back against Ultraseven by spitting a foam-like liquid from its mouth that weakened him, but the tides turned in Ultraseven’s favor when an injection from the Terrestrial Defense Force’s medical center began to restore Kaori’s physical condition. Ultraseven recovered and damaged Darii with his Emerium Beam and then finished Darii off in mid-air as it fell down from Kaori’s cell walls using his Ultra Bubble attack.