Before the bombs dropped, Desmond was a player in international intelligence at the highest levels. Desmond's motives and obsessions have been scarcely affected by the apocalyptic devastation of the planet. Desmond and those he would consider his peers knew for some time that the Great War was inevitable. Desmond himself could have been directly involved in the actions leading up to the nuclear exchange.
Facing the nuclear holocaust, global leaders and members of the intelligence community made their personal bids for survival in the new world. F.E.V., cryogenic stasis, AI consciousness, even - in Desmond's case - controlled exposure to normally lethal doses of radiation were among the wildly risky and experimental techniques pursued in the name of survival in a world without modern infrastructure. Old rivalries and vendettas were not forgotten in the ashes of nuclear winter. Those who managed to survive quickly set about doing what the bombs had not: wiping out any remnants of the Old World that may be a threat to personal and political agendas. Now, even so long after they should all have died naturally, these scattered figures seek power and jealously guard their secrets. Desmond's mission has long since ceased to be political or idealogical. He knew - as experience has proven tenfold - that as long as any of his colleagues lived, he would be hunted. His purpose is simple: kill or be killed.
He was responsible for ridding Calvert of a body, and due to this the two have a raging vendetta against each other that can only be ultimately resolved by one of their deaths.
Some of his dialogue hints to Desmond being British. His accent and dialogue phrasing seem to confirm this. His general accent and use of a soft "a" when pronouncing words such as "bastard" and "fast," indicate he is from the South or Midlands. Professor Calvert also refers to Desmond as "limey," a semi-derogatory term for the British. Another hint is that when you ask him about Nadine he will use the term "bird" which is an informal way of saying girl used in Southern England.
If kept alive, Desmond disappears from the game after exiting the underground lab upon completion of A Meeting of the Minds. If talked to beforehand, he will mention traveling north to pursue his next rival, another "player" in the "Great Game," a Post-War struggle between former geniuses and billionares.
Even though Desmond is rude, he appears to be the "Good" Karma choice for the Meeting of the Minds quest, since he drops an ear if killed by the Lone Wanderer with the Contract Killer perk. However, Karma is not lost for killing him.
Every time Desmond is knocked out before the end of the quest A Meeting of the Minds, his body spawns an ear which can be pickpocketed from him.
Despite the Lone Wanderer helping Desmond out and doing his "dirty work," Desmond stays rude to them even if they decide to help him out all the way. He will, however, say quietly, "It pains me to say it, but thank you..." if you walk near him after killing Calvert.
The only way to obtain all unique items in Point Lookout requires killing Desmond after completing A Meeting of the Minds in his favor.
When explaining where Calvert is hiding, he says "right here" even though his dialogue text says "the lighthouse," referring to the Point Lookout Lighthouse, which is virtually right there.
"What's the difference between you and me, kid? What makes a gifted killer like yourself into a rock-hard bastard like me? The answer is TRAINING you ignorant FUCK! I've got a hundred years of experience on you, and don't you forget it!"
ps3 Upon his death, Desmonds' corpse will have two ears on it, rather than just one. [verified]
ps3 Desmond tells you to lay mines for the invading tribals, however if Desmond approaches them they will detonate and Desmond will become hostile towards the player. Hiding from Desmond and waiting while Hidden will rectify this. Desmond will calm down and return to normal. [verified]