Viluy (Byruit in the dub) is the 4th member of the Witches 5, named for the mineral viluite.
In the anime, Viluy was handed the task of gathering pure hearts after Tellu's demise. To achieve this goal, she programmed all the computers at the Infinity Academy to extract the pure heart crystal of their users during a national practice exam. Additionally, she possessed the "Mosaic Buster" attack ("Mosaic Energy Blaster" in the English Dub), which released a flurry of nano-robots from her wristband. She had great success at first, and nearly defeated Sailor Uranus, Sailor Neptune, and Sailor Mercury (against whom she had a personal grudge), when her wristband suddenly malfunctioned as a result of damage inflicted by Sailor Moon attacking it. The nanomachines dissolved their own master, effectively killing her, leaving behind only her bracelet.
In the manga, Viluy was the third witch to appear, before Tellu. She was ranked at Level 202 and led the Science Division of Infinity Academy under the alias of Yui Bidoh. Her story in the manga is essentially the same as in the anime, except that she meets her doom by being cut in half by Sailor Uranus's Space Sword.