The Exos were the brainchildren of Clovis Bray,  though the motive for their creation remains a mystery. During the Collapse, the Exos' memories were shattered  and nigh-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 built during humanity's Golden Age for a long forgotten struggle, possibly against the Vex, with the purpose of protecting their creators. 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. But there is one clue to their origins; many Exos experience a dream they call 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."
The Exo is a robotic body with an uploaded human consciousness. The technology to encode the latter into the former was developed by Clovis Bray. This existence is not naturally sustainable; early Exos suffered from a condition known as Dissociative Exomind Rejection (DER), a serious and ultimately fatal cognitive disorder caused by the human mind rejecting its new body.
Clovis Bray discovered that DER could be staved off by two particular methods. The first was the "reboot", erasing the mind's memories post-transfer so that it would not remember its old body and so better accept the new one. 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". Successive reboots cumulatively eroded an Exo's mind and their ability to retain any memories at all. Clovis Bray estimated that the maximum number of times an Exo could be rebooted was around 20. Not all reboots completely remove past memories, though, especially for Guardians and some can gain access to these buried memories by purposefully dying or killing themselves. Every Exo has a number appended to their name that signifies the number of reboots they have experienced.
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.
Exos appear unyielding and tireless. 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.
List of appearances