Mithras is a sun deity who was worshiped in the Roman empire from the 1st to the 4th century AD and is the patron of Mithraism. He was said to be reborn after death, and a festival was held on the winter solstice for him. It is believed that Mithraism was popular in the Roman military and did not accept female disciples. Whether Mithras was directly inspired by the much older Mithra from Persia or Mitra from ancient India remains debatable. It is today considered a mystery religion due to lack of textual records of the cult, but many Mithraic temples and artifacts survive.
He is said to be appear as a being born from a rock as an adult and holding a knife which would be used for bull-slaughtering, a ritual believed to be commonly practiced by the disciples in honor of the god.
"A sun deity who was worshiped in the Roman empire from the 1st to the 4th century AD. The rituals of the religion were secretive, but it is believed that they had to do with covering oneself in the blood of sheep and bulls. He was said to be reborn after death, and a festival was held on the winter solstice for him."
—Shin Megami Tensei: Strange Journey compendium
Mithras is the master of Sector Bootes and is introduced as Master Mitra, or simply Mitra, appearing in a form possessed in earlier titles at first. He and his servants have been "recruiting" members of the expedition by threat or force and appears to be interested in the protagonist and offers him a chance to join him in return for being made a citizen of Mitra's domain in the new world to come, an invitation he extends to everyone aboard the Red Sprite as well.
The Red Sprite crew uses the Luxury Bell Mitra provided the protagonist with and creates a diversion allowing him to reach the higher levels of his tower and do combat with him. Upon his defeat, he drops Bootes' Rosetta and a Rare Forma. Later, when traversing Sector Delphinus, it becomes apparent Sectors Bootes and Delphinus have been collaborating for extended periods on human experimentation, as seen when upon reaching Mitra's private laboratories, a trophy, the Madman's Stone, has a plaque explaining the bond between Mitra and Asura, Delphinus' master.
He later reappears in Fornax and transforms into the lion-headed "Mithras" to seek revenge on the protagonist.
Shadow Kazuya Makigami takes the form of Mithras during the request, "Phantom Thieves VS Burglary Ring." He will summon an Oni and Onmoraki to aid him in battle, and will also attempt to buff them with Tarukaja.
Mithras is the form Kazuya Makigami's shadow takes upon learning that the Phantom Thieves of Hearts have come to steal his heart. He summons Oni and Onmoraki to defend him and sacrifices them when the two are no match for the thieves.