Osiris is a legendary Warlock and former Vanguard Commander who was exiled from The Last City due to his obsession with the Vex. During his exile, Osiris continued his research on the Vex and discovered the Infinite Forest on Mercury, which he proceeded to explore to thwart the Vex’s plans for the Solar System.
After being aided by The Guardian and Ikora Rey in stopping Panoptes, Infinite Mind, Osiris reestablished connections with the Last City and continued exploring the Infinite Forest while seeking the means to save Saint-14 from his death within the Vex machine. After succeeding in saving his partner with the use of the Sundial, Osiris and his Ghost, Sagira, departed Mercury to gather allies and information about a dark future he saw approaching the solar system.
Most recently, Sagira was killed in action during a conflict on the Moon. It was later revealed that the Osiris that had been assisting the Vanguard during the Wrathborn, Cabal Conflict and the rise in Xivu Arath forces had been none other than Savathûn herself. The Guardians eventually made a trade for Osiris by helping rid Savathûn of her Worm.
Information extracted from Destinypedia and Ishtar Collective. To view lore entries from Ishtar Collective, click here.