She started her career as an iconoclastic Guardian, with a reputation for being outspoken
and impatient with the dogma of her peers. She had a very colorful life on the battlefield,
whether fighting in the Crucible or doing solo missions and getting shot down repeatedly,
surviving against all odds. Her experience gave her a wealth of knowledge and newfound respect
among her fellow Warlocks.
Eventually, Ikora became a student of the Warlock Vanguard Osiris, arguably his greatest one. However, after her mentor's arrogance and obsession with the Vex grew out of control, she was forced to secretly take over many of his duties as Vanguard. Eventually, his focus on the Vex caused the Last City's other leaders to convene a meeting of the Consensus without Osiris present. The Speaker asked Ikora to take over officially as Warlock Vanguard, which she accepted. The Speaker then announced that he was exiling Osiris for his failure to serve the Last City and putting Guardians lives at risk chasing after Vex secrets. With no objections, Osiris was officially exiled and his post as Vanguard Commander was taken over by Zavala.
Cayde used to be a human soldier with a large debt over his head, when he was approached by Clovis Bray to have his debt removed. The job offer itself isn't explicitly stated, but what Cayde remembers next is coming to Europa where he was converted into an Exo. Upon becoming an Exo, Cayde spent some amount of time in the Eventide colony on Europa, at one point coming across Micah Abram as the boy tried to sneak into Bray Exoscience; Cayde had flashbacks of the encounter even after his five subsequent resets. Cayde witnessed the Collapse, remembering being restrained by a shadowy creature on a destroyed world.
While the City was preparing for an assault by the combined forces of all the Fallen houses, tensions between the Speaker and Vanguard Commander Osiris reached a breaking point. Zavala, having recently become the Titan Vanguard by replacing Saint-14, attended a meeting of the Consensus without Osiris present. When the New Monarchy representative protested that Osiris was absent, Zavala announced that Ikora would be filling in for Osiris. The Speaker asked her to formally become the new Warlock Vanguard, which she accepted. The Speaker then proclaimed that he was exiling Osiris from the Last City and thus named Zavala as the new Vanguard Commander in place of the troublesome Warlock.