The Nio (Benevolent Kings) are a pair of protectors who stand guard outside the temple gate at most Japanese Buddhist temples, one on either side of the entrance. In Japan, the gate itself is often called the Nio-mon (literally Nio Gate). Their fierce and threatening appearance wards off evil spirits and keeps the temple ground free of demons and thieves. In some accounts, the Nio were said to have followed and protected the historical Buddha when he traveled throughout India. They have since been adopted by the Japanese into the Japanese Buddhist pantheon. Each is named after a particular cosmic sound. The open-mouthed figure is called "Agyo," who is uttering the sound "ah," meaning birth. His closed-mouth partner is called "Ungyo," who sounds "un" or "om," meaning death. Other explanations for the open/closed mouth include: (1) mouth open to scare off demons, closed to shelter/keep in the good spirits; (2) "Ah" is the first letter in the Sanskrit alphabet and "Un" is the last (same in Japanese syllabary too), so the combination symbolically represents all possible outcomes (from alpha to omega) in the cosmic dance of existence. At some Buddhist temples, the Nio guardians are replaced with a pair of mythical and magical Shishi Lion-Dogs -- one with mouth open, the other closed.
These nio were the symbols of the Arakaki Naha Te Kempo DoJo that I trained in while living in Okinawa. I used the Nio as my DoJo's symbol when I returned to the U.S.
To me, the nio symbolize guardian angels, who protect us from the adversary, Satan, his demons, and ourselves, by reminding us of our true purpose, when we are not thinking or acting righteously.