The Ni-Ou (仁王*)?, also known as the Kongorikishi, are two protectors of Buddha. They are portrayed as overtly muscular and filled with anger and wrath; however their function is to protect the Buddha. One is named Agyo (or Kongo Naraen), and is depicted with his mouth open. The other is named Ungyo (Kongo Misshaku), and is depicted with his mouth closed. They are both said to be manifestations of Dainichi Nyorai.
"Origin: India. Guardian of entrances to Buddhist temples. Was originally the diamond wrestlers A and Un. Shuukongoushin, split into Misshaku Kongou and Naraen Kongou, is considered to be Niou."
—Shin Megami Tensei A-Mode DDS Dictionary
Niou is a boss fought in the Sugamo Prison of Ikebukuro if they are found to be guilty and locked up by Yama when brought before him by the Chaos Hero.