“You're not supposed to be in here! You're going to ruin the surprise!”
The following article/section contains spoilers. Please complete The New War and The Duviri Paradox before proceeding.
The Drifter is a playable character as a version of the Tenno from an "unrealized possibility", or alternate reality. They are first seen in either The Duviri Paradox or The New War depending on player choice. The Drifter was initially introduced during The New War in Update 31.0 (2021-12-15).
Ever get that feeling you're stuck? Spinning inside your little wheel. Only to find... you're right back where you started.
And so it goes. Day by day, year by year, until you're dead... inside. You feel nothing. You're just a prisoner trapped in the spiral of someone else's dream...Trapped in the Void's Duviri in an endless loop failing to escape from the mad ruler Dominus Thrax, one day the Drifter is mysteriously stabbed followed by a strange hand bonding itself to the Drifter and granting them powers and the ability to control Warframes that have arrived from beyond the Void. The Drifter is led by the hand to Teshin, who offers his 
Sun & Moon and teaches them the ways of the Warframe, explaining the origins of the Tenno and the Zariman Ten Zero ship in Duviri's background. They recover a toy figure, which Teshin believes will help them escape Duviri, while peering into Dominus Thrax's emotional states that occurred on the Zariman.
Meanwhile, Dominus Thrax grows increasingly frustrated and eventually has Teshin captured and killed, and dies before the Drifter. They take control of an Orowyrm and raid Thrax's throne, completing the figurine. Recovering the final memory, they discovered Duviri was inadvertently brought to life from the Tales of Duviri storybook through Conceptual Embodiment not by Thrax, but the Drifter, as a result of their emotional anxieties aboard the Zariman. They seize the throne and use its time-rewinding powers to resurrect Teshin but put Thrax back in power, which the Drifter deems to be a fair trade.
No longer shackled by Duviri, the Drifter is now able to leave for the Origin System to repay the aid from beyond the Void's walls. Teshin warns that once they cross over they will not be as who they are now, but as they were then.
The Drifter eventually makes their way into the Origin System during The New War, which has now been thrown into disarray following Ballas and Narmer's takeover. During the process, the Drifter subsequently lost access the strange hand bonded to them, removing their borrowed capability to Transference like in Duviri. They work together with Ordis to revive the Lotus, and form an unlikely alliance with Hunhow to combat the Archons.
While reviving the Lotus, she fails to recognize the Drifter and turns hostile. They flee long enough for Eternalism to catch up, granting the Drifter Void powers while restoring the Operator, prompting the Lotus to leave. The Operator heads to the Zariman Ten Zero and meets with the Drifter, who explains that they are from a timeline where they were not rescued and were not given the powers of Transference and Warframes. However, the paradox only allows either one of them to continue on to end the threat of The New War.
You wouldn't welch on a deal, would you?
Within one of the scattered Zariman Tablets discovered in Duviri, there exists an entry implying the existence of an instance during Zariman Void Jump, where the player Tenno accepted their doppelgänger's deal to save everyone, but was the only one not rescued in the process. The exact process of the saving of the other children is left unknown, but the child would be left alone in the Zariman ship adrift in the Void, with no Tenno power inherited from the deal. The adrift child in the ship would soon resort to the Tales of Duviri as a coping mechanism, their emotions and memories reacting with the Void, bringing the Kingdom of Duviri to life via Conceptual Embodiment as an archipelago floating close to the Zariman. That version of the child would become the Drifter.
The Operator and Drifter meet the Lotus in Sanctum Anatomica. She warns them that she is being influenced by The Man in the Wall who is targeting the former, thus she chooses to send the latter to 1999 and urges them to find Albrecht Entrati.
Sirocco model, even if the skin itself is not equipped.