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Unlike mass-produced robotic frames, Exos are human minds uploaded in artificial bodies; with the notable exception of Felwinter, each and every Exo used to be a human being. Before uploading technology was developed by the Clovis Bray corporation during the Golden Age; many Exos come from the mysterious Deep Stone Crypt on Europa. Exo names typically include the number of memory-wiping "reboots" of their brain, e.g. Cayde-6 or Saint-14. While not being biologically "alive", Exos retain many human qualities and habits; just like any living being, they have Light of their own. They can die and be revived by a Ghost.
The Exos were the brainchildren of Clovis Bray, , who originally developed them as a means for humans to attain immortality. During the Collapse, the Exos' memories were shattered  and nearly all knowledge of their origins lost as well. In the Dark Age that followed, the Exos helped build the Last City alongside their organic brethren, with many going on to become Guardians. 
The Exos were once involved in an ancient conflict, and Rasputin was once involved in their command, a fact that some Exos dimly remember. The conflict itself has long been forgotten and the Exo remain ciphers, even to themselves, for their purpose has been forgotten as well. However, there is one clue to their origins; many Exos experience a dream they call the Deep Stone Crypt, or the "place where they were born." It is described as "the infamous subroutine which seeded the first Exo consciousness." Cayde-6's memories stretch further, where he recalls being a human before travelling to Europa to become an Exo, as he "came there in flesh and bone. Gave everything to the ice. Started over. Rebooted."
Exos appear unyielding and tireless, to the point that appearing exhausted is often considered impressive for an Exo. Mechanically, they are so advanced that only a Ghost can comprehend their inner workings. In many ways, Exos like to think of themselves as human, or having human minds in machine bodies. Exos can also confuse armor they wear for part of their own body, which leads to a feeling of being naked.
Exos are subject to a number of unique medical conditions, being Dissociative Exomind Rejection (DER) and spontaneous Exo reset syndrome. The former is a serious and ultimately fatal cognitive disorder caused by the human mind rejecting its new body, and the latter is an involuntary self-reset that can occur without any immediately apparent cause or warning.
The Exos were designed and built during humanity's Golden Age by the Clovis Bray corporation. The original intent of the project was to develop a durable, synthetic housing for an uploaded human consciousness (an "exomind"), which would effectively render humans immortal. However, early exominds inevitably fell victim to a degenerative process termed the "loop/billboard/crash" cycle, characterized by cognitive and motor stereotypy (abnormal, repetitive behaviors), athetosis (slow, involuntary writhing movements of the limbs and extremities), and gradual loss of sapience. While degenerating exominds would still pass the Turing test for a time, they were philosophical zombies, and would ultimately enter a state analogous to brain death. This problem eventually claimed the life of Clovis Bray II, after he was uploaded into an exomind format. Crashed exominds could be restored from backup, but the process termed "billboarding" would simply begin again.
Following the discovery of the K1 artifact and the paracausal force Clovis Bray I called "Clarity", Clovis I mounted an expedition to Europa to find the secret of immortality promised to him by "Clarity". There, under the instructions of a mysterious entity he called "Clarity Control", Clovis I stole a Vex from the Ishtar Collective and used it to build a gate to a Vex megastructure, the Forge Star. Upon studying samples of Vex radiolarian fluid recovered from the Forge Star and observing its synergy with "Clarity", Clovis I discovered that introducing "Clarity"-altered radiolarian fluid, deemed "Alkahest", to an exomind's hardware miraculously solved the "loop/billboard/crash" problem. However, even this solution was not permanent, as the new generation of Exos fell victim to Dissociative Exomind Rejection.
Clovis I discovered that DER could be staved off by two methods. The first was the "reboot", which involved wiping the mind's memories post-transfer so that it would not remember it's old body, and so better accept the new one. Despite the term "reboot", also referred to as "resets" or "memory wipes", the process did not actually remove memories, and only blocked off associative access to them. This was done because memory engrams were stored in many areas of the brain, causing them to be difficult to locate and delete. While effective, it was not a cure, and the reboot procedure would often be repeated several times over the Exo's life. It would also be used for functional purposes, such as restoring Exos whose minds had become "compromised," such as by Vex infection. Successive reboots cumulatively eroded an Exo's mind and their ability to retain any memories at all, with excessive numbers of reboots causing insanity. The Clovis Bray corporation estimated that the maximum number of times an Exo could be safely rebooted was around 20. As reboots did not technically remove any memories, Exos sometimes remembered traces of them. This was especially the case for Guardians, and some could gain access to these buried memories by purposefully dying. By default, Exos had a number appended to their names that signified the number of reboots they had experienced, with the original human used as the zero-index. Hence, to use Clovis I's example, the human John would be followed by the Exo John-1, John-2 after one reset, etc. This number was stored in the Exo's memory as hardware, and so could not be forgotten.
The second method Clovis Bray used to prevent DER was to introduce familiar organic conceits into the Exo's mechanical body, such as the drive to eat or reproduce. While these "humanisms" were not functionally necessary, they nevertheless reduced the chances of DER developing, as without routine feedback from bodily processes the Exo would become convinced those processes had failed and develop severe Cotard delusion. The "humanisms" may extend to certain sensations of discomfort, such as nausea or physical exertion, as Cayde-6 has demonstrated on occasion, though he is not a reliable source. It is not possible for an Exo to become intoxicated by alcohol, however.
During the development of the Exo creation process, Clovis Bray founded the Europan colony of Eventide to house clients awaiting uploading, and the resulting Exos.
After the Collapse, many Exos survived and would go on to participate in the founding of the Last City. Many would also be resurrected by the Ghosts to become Guardians. Even among non-Guardian Exos, virtually all memories of their origins and the Collapse appear to have been lost over the course of multiple memory wipes; an exception is Ada-1, whose memory has never been wiped.
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