Desmond Miles is the true protagonist of the Assassin's Creed franchise, though he is not the primary playable character. Desmond was to be raised as an assassin. However, he escaped his parents to see the rest of the world, later becoming a bartender. He is voiced by Nolan North.
During Desmond's childhood, he and his family lived in a secluded assassin hideout known only as "the Farm". There he was put into serious training by his father, William Miles. He always questioned the Assassin-Templar conflict, believing the conspiracy to only exist in the imaginations of the paranoid adults of the Farm. Then one day, he was fed up with his mother and father, calling them crazy, and ran away from the Farm. Soon after running away he did small jobs in New York to earn money. Eventually, he was working in a bar as his first serious job. As an adult he often joked of the conflict, unable to take anything related to the words "Templars" or "assassins" seriously, going as far as to invent a drink called a "Shirley Templar" which is just a Shirley Temple with an added dose of gin.
In 2012, Desmond was captured by henchmen working for Abstergo Industries for the Animus Project, which centers around Desmond's minder, Dr. Warren Vidic, subjecting him to the Animus, a machine that extracts a person's genetic memories regarding their ancestors from their DNA. This is where Assassin's Creed begins. It is here that he discovers he is a descendant of Altaïr Ibn la-Ahad, a legendary assassin who lived during the Crusades and worked to defeat the organization known as the Knights Templar. Throughout his captivity, Desmond is subjected to the Animus by and he learns that Abstergo Industries is the modern face of the Templars, who still exist; his parents weren't crazy and the secret conflict for the fate of the world was real. Abstergo was searching the memories of Altair, attempting to find an artifact called a Piece of Eden. After the Animus reveals a map to the Pieces of Eden, Desmond discovers he has somehow absorbed "Eagle Vision", a supernatural trait possessed by Altaïr, which allows the user to identify at a glance who is an enemy, target, and ally. Desmond, however, uses it and discovers invisible hieroglyphics and strange symbols painted all over the walls and floors around the Animus machine. He goes to his room and discovers more strange writing above his bed, which he believes to be blood which turns out to be written by Subject 16, the previous person in Desmond's position, who went insane from prolonged exposure to the Animus. Around the same time, Lucy Stilman, assistant to Dr. Vidic, reveals herself to be an Assassin, working undercover at Abstergo to fight them from within. She implies that she will help free Desmond when the time is right.
Assassin's Creed II
Picking up immediately where the first game left off, Assassin's Creed II sees Desmond and Lucy fleeing the Abstergo Industries compound in order to reach a secret Assassin base, but before setting off, Lucy uses the Animus to find the most recent common ancestor of Desmond and Subject 16, which turns out to be another Legendary Assassin, a 16th century Italian nobleman named Ezio Auditore da Firenze. Later, inside the resistance base, Desmond will have the opportunity to utilize an updated "Animus 2.0" which utilizes the "bleeding effect" (which is how Desmond absorbed Altair's Eagle Vision ability) in a controlled manner to allow the user to absorb the skills and instincts of their ancestor through spending time in their shoes. This will allow Desmond to absorb Ezio's master Assassin skills in a matter of days. Additionally, the updated Animus allows Lucy, as well as fellow Assassins Shaun Hastings and Rebecca Crane, to keep in contact with Desmond whilst he accesses a genetic memory.
After spending a long time on the Animus 2.0, Desmond was told to take a break and use his new-found skills to set up the security system. After that he was returning to the group when he had a flashback from Altair's time without the help of the Animus, in which he witnessed the end of Altair's genetic memories: the moment in which he impregnated Maria Thorpe, the love of his life, fathering the next of Desmond's ancestors. After his flashback, Desmond returned to the Animus 2.0, where he learned through Ezio that the Pieces of Eden were the creation of the "First Civilization", a race of advanced superbeings who were wiped from existence along with the majority of early Humanity when an unexpected solar flare struck the Earth thousands of years ago, incinerating vast swaths of land. Unfortunately, the solar flare is cyclical, and as of 2012, only a short time remains before the Earth will be devastated again, unless the Assassins can do something about it.
Later, after Desmond completed Ezio's story, Abstergo broke into the hideout and Desmond had to help Lucy defend the base. Desmond was given a hidden blade and fought off the Templars. Desmond and Lucy escaped with the rest of the group and headed to another hideout. While reaching the next destination, Desmond went back into the Animus and trys to unravel more clues.
Assassin's Creed: Brotherhood
After the events of Assassin's Creed II, Desmond and his crew of Assassins travelled to Monteriggioni, the small town which had been the headquarters of Ezio's band of Assassins during the events seen in Assassin's Creed II. Because the Templars were searching for them through cell phone towers, they decided to set up the hideout underground in the Sanctuary underneath the Villa Auditore. While searching for a way into the Sanctuary, Desmond experienced the negative side effects of the bleeding effect, as he sees visions of Ezio running around and escaping from the attack on the Villa. As a result of these visions, Desmond became worried, believing he might turn out like Subject 16. By the time Desmond and Lucy reach the sanctuary, Desmond experienced another vision, this time of a much older Ezio. As Desmond tried to open the door to let the other two in, he noticed the numbers 1419, 1420, and 1421 written on the walls, which Shaun believed to be dates. After Desmond brought power to the Sanctuary, he continued exploring Ezio's memories inside the Animus 2.0.
After Desmond finished with the Animus, the Assassins looked at the numbers on the walls again and realized that the Apple of Eden Ezio had obtained was hidden in the Colosseum of Rome. They reached the Colosseum, and as Desmond tried to find a way for the others to follow, he twice encountered a hologram of Juno, a member of the First Civilization, like Minerva, who had spoken to Desmond through Ezio in Assassin's Creed II. Inside the Santa Maria Aracoeli, the Assassins activated an elevator that led them to the Apple's chamber. As Desmond grabbed the Apple of Eden, Juno told Desmond that his DNA activated the Apple, and took control of his body, freezing the rest of the assassins in place. Juno forced Desmond to walk towards Lucy and spoke, saying that there is one who would "accompany him through the gate" and that "she lies not within our sight". Despite his struggles to resist, Desmond is forced to stab Lucy in the stomach with his hidden blade and goes into shock as his consciousness is shattered. During the credits, there is a brief conversation between two unknown men as they put Desmond back into the Animus, where he fades into the "safe room."
Assassin's Creed: Revelations
After stabbing Lucy, Desmond fades into a coma and is plugged into the "black room", a virtual construct of the Animus 2.0, to keep him alive, with the net effect that he is fully conscious as a digital being inside the Animus, although if he doesn't find a way to restore himself to his body soon, his physical body will die and he will be trapped in the Animus forever, the fate of Subject 16. Upon waking up, Desmond finds Subject 16 on "Animus Island" with him, telling him he must explore Ezio's remaining memories to separate his ancestors' memories from his own to be able to survive. Desmond thus explores Ezio's final grand adventure, a search for ancient relics in Constantinople, talking with Subject 16 along the way. On occasion, upon returning to Animus Island, Desmond can hear bits of conversations occurring next to his body in the real world; through this he learns that Rebecca and Shaun have returned him to a major cell of the Assassins, lead by his father, and that Lucy did indeed die of her wounds. Eventually, the Animus detects Desmond's consciousness and sees it as an invading computer system, promptly trying to delete Desmond, Subject 16 sacrifices himself for Desmond so he can save himself, but mysteriously leaves behind a piece of code within Desmond's consciousness. Desmond reaches the "Sync Nexus", freeing himself of the psychological downsides to the bleeding effect. There, he receives a heartfelt farewell message from Ezio, and is given a vision by the First Civilization, showing him the destruction of their world by the solar flare, and telling him to go to the Grand Temple, showing him the location somewhere in the United States of America. Desmond then wakes up from his coma, saying he knows what they need to do.
In the inexplicably optional "The Lost Archive" DLC, Subject 16 unlocks a stolen Abstergo file for Desmond, and shows him the uncomfortable truth of "Project Siren"; Lucy Stilman was a triple agent. She volunteered for the job of infiltrating Abstergo because she believed the Assassins were never going to defeat the Templars, and promptly defected as soon as she was inside. Every facet of her relationship with Desmond and their "escape" from the Abstergo facility was planned in advance with her mentor, Dr. Warren Vidic, and the combative nature of their relationship as seen in the first game was a show put on for Desmond's benefit. Ultimately, Lucy had been planning to play the part of the ally to Desmond, going as far as to imply romantic interest, until he uncovered the location of another Piece of Eden. She had been planning to betray Desmond and the Assassins to Abstergo as soon as they retrieved the artifact. The only thing that prevented this plan from succeeding was Juno forcing Desmond to murder the woman he thought was his friend, causing Abstergo's Project Siren to fall apart at the last minute.
Assassin's Creed III
In Assassin's Creed 3, Desmond finally finds the precursor cave where the answers as to how the world can be saved are hidden. However, Desmond ends up having to go and relive his ancestor Connor Kenway's memories in order to figure out how to open the giant vault door that stands between him and possible salvation. In between trips inside the Animus, he goes on missions around the world to find power sources that might open the door. While travelling the world, he runs into Cross, a Templar assassin who's also looking for the power sources for Abstergo.
During the third and final power source mission, Desmond's father is kidnapped and taken to Abstergo's headquarters. Desmond goes to the Abstergo headquarters, kills Cross, frees his father and uses the Apple of Eden to make everyone at Abstergo to commit suicide thus destroying Abstergo once and for all.
Through Connor's memories, Desmond succeeds in opening the vault where he's confronted by both Juno and Minerva and a sphere that if he touches it would unleash Juno onto the world but also save the world from annihilation from the incoming solar flare. Minerva warns him that touching the sphere would lead humanity to be enslaved by Juno. Minerva also states that the flare wouldn't kill all of humanity and it will be possible to rebuild. Juno warns that doing this would send humanity back in a cycle. Desmond would become a Christ-like figure among the survivors and that after his death, his words would be taken as truths, like a religion. Eventually his words would be misinterpreted and it would lead to global oppression. Desmond decides to go with Juno, as the loss of life would be too great if he were to go with Minerva's option.
Desmond touches the sphere, which instantly kills him, unleashing Juno onto the world.
Desmond holds a serious view on life due to being raised as an Assassin since birth and not getting to see the rest of the world, after escaping the farm, Desmond goes incognito for a while, showing his skills at avoiding detection, he is eventually caught by a motorcycle license. Desmond however occasionally makes jokes to both friends and enemies. After escaping Abstergo, Desmond became more relaxed around friends and became more humorous and flirtatious, however after discovering the bleeding effect, he became worried for his sanity.