Excalibur

Every Tenno meets Excalibur early. He is the clean idea of a Warframe before the archive complicates it: a warrior with a sword and a gun, the shape the word makes in the mind. He is the face on the recruiting-plate, and for most of the game's history that was all he was permitted to be — a silhouette, an ideal, a frame with no wound of his own.

The Leverian keeps no exhibit for him. Drusus hangs his plates for the strange and the late-come, and Excalibur is neither. So the archive keeps this record in the Leverian's stead — because the frame that looks most like a legend turned out to carry the ugliest truth in the arsenal. (His first Prime, the very first of the Primes, is a separate matter of record: sealed behind the closed Founders program and now beyond any hand that did not already hold it.)

The Sacrifice pulled the truth into the open through one frame, and it is not a machine's truth. A Warframe is a person. The Orokin cultured a controllable strain of the Infestation — the Helminth — and used it to remake human subjects, willing or not, into hybrids fit to meet the Sentients. Executor Ballas built the program. Beneath the armour of every frame is someone the Empire unmade.

Excalibur Umbra is the proof and the indictment. He was a Dax, an oath-bound Orokin soldier, and he caught Ballas in his treason. Ballas answered not with death but with design: through a corrupted Transference, he made the Dax kill his own son, Isaah, as the boy was taken by the Infestation — and set that killing as the frame's one looping memory, a grief with no floor. Where other frames were emptied, Umbra was left awake inside the weapon. That is why he can move with no Tenno at the helm, and why, once the leash slipped, he turned on the man who made him.