Osiris

Osiris is an old Warlock who was exiled from The City, due to his obsession with the Vex and the Infinite Forest, and was said to have "come too close" to understanding the Vex. He has knowledge about the inner working of the Hive, as he knew about the Shrine of Oryx.

Exile
Osiris was exiled from the City due to his obsession with the Vex. The Titan Vanguard, Saint-14, searched for Osiris after Osiris's exile, but was unsucessful.

Return
The Guardian is summoned by Ikora Rey, who shows him Sagira, Osiris's Ghost, and shares a report that Vex activity has surged dramatically. The Guardian goes to Mercury, where a member of the Followers of Osiris, Brother Vance, awaits their arrival. Unfortunately, they were unable to enter the gateway that led to the Infinite Forest. They then go to Earth to activate Sagira at an Osiris base, but the brothers there were killed by Fallen before they arrived. Luckily, the ghost shell is revived when the Guardian gets close to a certain book. Somehow, Sagira is then merged with the Guardian's Ghost, taking control of him. The player soon returns to Mercury, where they are now able to pass through the gateway to the Infinite Forest. After missions guide the player through preparation and a final battle with Panoptes, Osiris extends his appreciation.

Rescue of Saint-14
Osiris instructs the Guardian to use the Sundial to rescue Saint-14, which they do.

Personality
Osiris is described as being an ego maniac.

Quotes

 * "Too long, my… I… I was going to say "student" - but, well, that word is too small for you now."

Trivia

 * There are two known members of the Cult of Osiris who still live: Brother Vance, and Sister Faora.
 * Osiris was Ikora Rey's mentor.
 * Osiris is the only Vanguard Commander to have been exiled, and has been referred to as "the Zavala before Zavala".
 * In Season of Dawn, Osiris becomes something like a vendor.
 * Osiris can only be found when you first enter the Lighthouse for a few seconds, but he quickly disappears shortly after you spawn in. He can be found talking with Saint-14 for a few short seconds, then teleporting away. Saint-14 congratulates you for your flawless run and states that "Osiris is off chasing down another loose thread." We know he is on a mission, and that he discussed the details with Saint-14, but we do not know any additional details, namely what or who the loose thread is.