Mithra is the antagonist of Moonlight Syndrome and by extension the Twilight Syndrome games as well, being the cause of the titular syndromes. Although he appears as a young child, Mithra is very violent, manipulative and cruel, killing Yukari Hasegawa, Arisa Kahara, Chisato Itsushima and Mika Kishi. He is killed by Ryo Kazan, who wields a katana to tear him apart in the conclusion of the story, apparently bringing an end to the syndrome.
Mithra also appears in a single scene in killer7 in the Alter Ego chapter, referred to as "The Oracle" in Hand in killer7. His reason for appearing here (apart from giving the killer7 a cryptic message about light and shadow) is unknown, though it may be connected to the full moon appearing at the beginning of each mission. As The Oracle, he appears as twins, one in front of the other, who share the voice of an old man.
In Zoroastrian mythology, Mithra is the god of contract, portrayed as a kind and benevolent figure. This makes his portrayal in Moonlight Syndrome a stark contrast to the original Mithra, appearing in the game as a ruthless villain. Later on, in killer7, he is portrayed in a more neutral manner; instead of forming a contract with the killer7, Mithra simply appears to give them cryptic advice, then disappearing just as suddenly.