In Japanese mythology, the deity depicted as the ruler of the underworld is Yama or Emma-o. Yama oversees the realm of the dead, ensuring the souls of the departed are judged. He is often portrayed as a stern figure, but also as a necessary part of the cycle of life and death. This duality of fear and respect reflects the cultural beliefs surrounding death and the afterlife in Japan, emphasizing the importance of properly honoring the dead for a peaceful transition.