Rell

Rell was a Tenno lost to history. The circumstances of his sacrifice are revealed in the Chains of Harrow quest.

General
Rell was one of a number of children aboard the Zariman Ten-Zero during the ill-fated voyage that created the Tenno. Unlike the other children, Rell found it difficult to identify emotions and would be overwhelmed by too much sensation. His mother Kay, another passenger aboard the Zariman, loved him dearly and worked patiently with him to manage his difficulties through a variety of tools, including a deck of cards with various faces drawn on them to help Rell identify emotions, or a humming, tuning-fork-like device called a "Donda" to help Rell focus his thoughts and deal with the sensations of sound, color, and touch which sometimes overwhelmed him. Kay was also unfortunately forced to defend Rell from the other children aboard the Zariman, some of whom mocked his afflictions.

During the incident that created the Tenno, Kay fell to the same mind-warping affliction as the rest of the adults on the ship. In her waning lucidity, she instructed Rell to wait with the other children "while the ship was fixed." However, as shown in his mini-comic, during the Zariman's drift through the Void, Rell was left out of the main group of Tenno and forced to fend for himself.

On at least one occasion, he was attacked by a "hunting party" of the adult crew and had to use his newfound Void abilities to protect himself. One of the adults who attempted to harm him was the now-maddened Kay. Rell stopped her with a Void Beam, possibly killing her, and simply stated that Kay was no longer his mother, that she was something else now - although what that was he hadn't figured out yet.

After Kay's attack, a Tenno girl attempted to bring Rell to one of the children's' safe-points only for a larger boy to physically force him out again, deeming Rell's "weirdness" to be a risk to the rest of the group.

Rell decided the group would have too much noise anyway; he would need his focus as he "had something to do, someone  to figure out." By the end of the Zariman's voyage, Rell became convinced that the Void itself was an amoral alien entity that existed before time.

After the Orokin recovered the derelict Zariman and retrieved the Tenno children, Rell was yet again "cast out" from the rest of the Tenno, possibly due to the Orokin deeming Rell unfit for the combat duties they forced upon the other Tenno.

Rell was eventually taken in by the precursors of the Red Veil. They studied his teachings about the dangers of the Void, eventually coming to call the Void entity he perceived as "The Indifference" or "The Lidless Eye". Rell preferred to call it "The Man in the Wall".

Denied access to Margulis's cryosleep and the Dream that preserved the other Tenno, Rell eventually and willingly sacrificed himself in a ritual the Veil believed would seal The Indifference out of the mortal plane. Rell's spirit was bound to his Warframe, Harrow, which was then chained in the Red Veil's sacred temple in a Derelict.

During the Chains of Harrow, Rell's spirit became splintered and separated from his vessel. While Tenno try to recover the fragments of his mind, Rell repeatedly mentions that The Man in the Wall is listening to the Tenno and grows stronger from that. Lotus and Ordis, however, explain it away as Void induced delusions and loss of control over Transference. Eventually Palladino, Speaker of The Veil, asks Tenno to destroy the vessel, thus freeing the weakened Rell, so he would be able to "rest", and placing the burden of holding off The Indifference on all remaining Tenno.