Osiris is a complex individual. We need to step back a few centuries to the period of time at the beginning of the City Age. He was a human male Warlock that was resurrected some time immediately preceding the Battle of Six Fronts. He was called upon to defend the City during the siege, and became known as a hero to the City due to his heroics at combating the Fallen during Six Fronts. It was during this time that he would meet the legendary Exo Titan, Saint-14, and the two would become close friends.
Following the Battle, Saint-14 would call for Osiris to be appointed as the Warlock Vanguard; with Saint-14’s good word on his side, the Vanguard appointed him to the position. Unlike the Vanguard today, Osiris was allowed to continue his crusades against the forces of the Darkness and personally lead many strikes against our enemies even after his appointment.
During this time, Osiris began to become a malcontent in regards to the established Hierarchy of the Tower. He began to question the specifics of Guardians and their histories before their resurrection to his close working partner, the Speaker.
Osiris specifically began to question why Guardians were so obedient to the Traveler and never questioned the orders of the Speaker and the Vanguard. Against the Speaker’s wishes, the Warlock Vanguard began to draw others to his cause, and many Guardians (including Titans and Hunters) began hunting for truths instead of defending our precious Last City. The Towers became increasingly divided at this point, and both Osiris and the Speaker distrusted each other deeply. Osiris began chasing after Xur and the answers of the Nine, dabbling in Thanatonautics, and even tracking down the remnants of the Ahamkara and their lore.
Then, Osiris left the City in exile. Whether this was of his own accord or if he was exiled by order of the Speaker or Consensus is not apparent to us. Either way, he departed the Tower and immediately left for Mercury. Now, centuries later, he has not made contact with the Vanguard or the Speaker, and most of the Tower’s leaders consider him dead. The Speaker sent his close friend, Saint-14, on a quest to find the lost Warlock, but the Exo Titan was never heard from again.
While Osiris may be gone, his adherents are not. His teachings and legacy are passed on by the Cult (or Disciples) of Osiris. The representative of which you know as Brother Vance in the Reef. Vance himself is a blind Warlock from the Tower. He claims that he left the City, and along with many other brothers and sister, continues on the teachings of Osiris through the Trials and meditation with Guardians that are patient enough.