He is the exiled 13th Guardian of Zodiac legend.

From http://zodiacbooks.com/13th-sign.html:

The legend of Ophiuchus’s downfall exists mainly as a common myth in Zodiac storybooks, told as a cautionary tale to all the children across the galaxy. Each House tells different versions of the story, but the most common form of the legend goes like this:

When the Thirteen Guardian Stars crashed onto planets in their own Houses, and they arose human, each with a Star Stone, or a Talisman, that stored their particular power—the strength they bring to the Zodiac. These mortal Guardians were now charged with harnessing the powers of their Stones to protect their planets and people—and with passing down the Stones to the inheriting Guardians after death.

Legend has it that the Stone belonging to Ophiuchus, the first and only Guardian of the storied Thirteenth House, represented Unity. A gifted healer, Ophiuchus was once the most welcoming and nurturing Guardian of the bunch. But eventually he became restless, and soon began to miss the immortality he’d given up in order to guard and protect the new human settlers. He explored his Talisman obsessively, knowing that its powers of Unity would eventually lead him once again to everlasting light.

When the other Guardians learned of Ophiuchus’s quest, they grew angry, and they felt he had overstepped his bounds as Guardian. They asked him to step down and pass on the Guardianship and his Talisman to a worthy human. When he refused, the Guardians retreated to decide what to do next. But during that time, Ophiuchus’s Talisman was stolen by an unknown thief; a citizen freedom fighter who cast the Unity Stone out into Space so no other Guardian could ever be tempted to make the same mistake.

Upon discovering his Talisman was missing, Ophiuchus received a message through the Psy from someone who claimed to have located it. The stranger didn’t say who he was, and when Ophiuchus met him on the astral plane, he stepped right into a trap. The stranger had found a way to bind Ophiuchus’s soul to a small corner of the Psy, where he couldn’t return to his body.

Without its Guardian or Talisman, the Thirteenth House began to fade and die. The other Guardians searched for Ophiuchus and his Stone, but they never found either. They took in most of the survivors from the Thirteenth constellation, who assimilated among the other Houses—but the die-hard Ophiuchans chose to go down with their world.

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