"All those hours locked under the stairs and you still weren't good enough. Not good enough for Daddy, not good enough for the Council" ―an Ethros demon to Wesley
Little is known about Wesley's childhood, only that it was a largely unhappy one. In the episode "Lineage", it is revealed that, at the age of six or seven, Wesley attempted to resurrect a dead bird using a mystical scroll stolen from his father's library. As a teenager he attended an English all-boys preparatory school where he became Head Boy while training to become a Watcher -- a member of a secret organisation that battles evil and trains Vampire Slayers.
Watcher to Buffy and Faith
Wesley comes to Sunnydale as a replacement Watcher for Slayers Faith Lehane and Buffy Summers, following the discharge of Rupert Giles by the Council. Clean-cut, pompous and bossy, he is immediately disliked by the girls under his watch. He is inexperienced and handles himself poorly in combat; prior to his assignment to Buffy and Faith, his only encounters with vampires had been brief and "under controlled circumstances". Giles, his predecessor and fellow Englishman, does not take him seriously, and tends to support Buffy's predilection for ignoring Wesley's counsel.
Originally Wesley was intended to be killed off after just a few episodes, but the writers grew fond of his character and decided to keep him alive. Wesley develops an inappropriate crush on Cordelia Chase, a high school student and ex-girlfriend of Xander Harris. Their flirtation culminates in a dance at the high school prom, followed a few days later by an awkward kiss, after which both characters realize that the attraction had been illusory and they lacked any sort of chemistry. When Faith goes rogue and Buffy turns her back on the Council, Wesley is subsequently fired for his failure.
Joining Angel Investigations
Wesley meets Angel and Cordelia again the next year, and he joins them as a member of supernatural detective agency Angel Investigations, serving as a partial replacement for the recently lost Doyle. Wesley proves his loyalty to Team Angel when, after being tempted with the offer of returning to the Watcher's Council, he betrays his former colleagues in order to help Angel protect Faith, despite the fact that Faith had brutally tortured him in the previous episode.
Wesley eventually finds himself running the organization in Angel's absence during his friend's descent into darkness and shows himself to be highly capable, as well as developing a strong brothers-in-arms relationship with streetwise vampire-hunter Charles Gunn. No longer the coward he once was, Wesley is shot trying to protect Gunn and his friends from a zombie police officer and remains in a wheelchair for several weeks afterwards.
After becoming the official leader to the group, Wesley seeks his father's approval over being made leader of Angel Investigations, but instead finds his father is more concerned about his next failure following his being fired from the Watchers' Council. This, in combination with the natural leadership displayed by Angel upon his return to the agency, results in a crisis of confidence for Wesley, who nevertheless re-asserts his leadership when he is placed in charge of a rebellion in Pylea, exhibiting a ruthless pragmatism when he explains his decisions to sacrifice some rebels' lives and risk Angel's life as well: "If you try not to get anybody killed, you end up getting everybody killed."
In the third season, Wesley's path soon becomes filled with tragedies and difficult choices. Just as he is beginning to have romantic feelings for teammate Fred Burkle, he is supernaturally influenced to attempt to kill her, ending his immediate opportunity for love (cf. "Billy").
After the birth of Angel's son Connor, Wesley becomes convinced by a prophecy that Angel will eventually kill the baby. With the intention of taking him to safety, Wesley betrays his friends by kidnapping Connor.Wesley's plan goes terribly wrong, resulting in his throat cut and the baby being taken to a hell dimension. As Wesley recovers, a furious Angel tries to kill him and he is alienated from his friends.
Wesley continues his descent into self-loathing when he begins a sexual relationship with Lilah Morgan, an employee of evil law firm Wolfram & Hart and Angel's long-time enemy. He forms his own team to fight the good fight, but maintains an interest in the affairs of Angel Investigations. In the opening episode of Season Four, Wesley rescues Angel from the bottom of the ocean, reviving him with his own blood. Wesley eventually returns to the team full time during the gang's first confrontation with The Beast and Jasmine's manifestation. Now deeply serious, humorless, and pessimistic, he takes command and makes the difficult decisions of bringing back Angel's evil alter ego Angelus and helping Faith escape from jail in order to stop the Beast. Wesley also has to face the trauma of Lilah's death, particularly when he is forced to decapitate her corpse when it appears she was killed and possibly sired by Angelus (cf. "Salvage"). Angel performs a powerful mind-wipe which removes certain events from the collective memories of the world at large.
Wolfram & Hart
As Wesley adjusts to working for Wolfram & Hart, his father arrives to try and convince Wesley to join the new Watcher's Council. Roger Wyndam-Pryce is soon revealed to have sinister intentions when he attempts to steal Angel's free will. When his father threatens to kill Fred, Wesley shoots and kills him without a moment's hesitation, and continues to fire his gun until the clip is empty. Although it turned out that the being he had killed wasn't his real father, but a convincing cyborg copy, Wesley is still understandably traumatized.
Wesley finally gets together with Fred, only to lose her shortly after, when her body is occupied by the spirit of the Old One known as Illyria. This tragedy leaves Wesley a broken man who turns to alcohol to ease his pain and tries to help Illyria understand the world, a move that furthers Wesley's suffering but, at the same time, he sees as the only way to keep some part of Fred close to him.
Upon finding out that Charles Gunn was indirectly responsible for what happened to Fred, he stabs him in retaliation. Wesley also kills Knox for his part in Illyria's ascension, a fact that annoyed Angel as he had just been giving a speech about how fighting for human lives was what made them better than the likes of Illyria.
When Angel proposes to attack the Circle of the Black Thorn, the secret arm of the Senior Partners, Wesley agrees to challenge a demon sorcerer named Cyvus Vail, who knows of Wesley's growing instability and believes him to be a wild card who could betray Angel. Vail proves too powerful for Wesley, and he is overpowered and mortally wounded. Despite his wounds, Wesley manages to temporarily knock Vail out with an energy sphere, which allows him to spend his final minutes of life with Illyria at his side, finally agreeing to let the demon take the form of Fred to allow Wesley, in some way, to say goodbye to the woman he loved.
After Wesley passes away, Illyria avenges his death by shattering Vail's skull with a single punch. We are never made aware if Wesley is subject to the same "standard" perpetuity clause as Lilah and Holland Manners that retains their service for the firm after death.
Following his death, Wesley is now a ghost due to the standard perpetuity clause in his Wolfram & Hart contract keeping him working for the company even after death. He is currently the liaison between the Senior Partners and the now Hell-based L.A. branch of Wolfram and Hart.
Wesley's most notable personality trait is his ruthlessness; he is "one to make the hard decisions even if he has to make them alone." This is evidenced even in his early episodes on Buffy, where he would rather allow Willow to die than to give the Mayor back an object which would allow him to complete his Ascension. He is immediately dismissed by the Scoobies as lacking in compassion, though his logic is sound (cf. "Choices"). Later examples include taking baby Connor away from Angel and shooting what appeared to be, at the time, his own father.
Powers and abilities
Wesley had average strength for a man his height and weight, although he was somewhat athletic due to the lifestyle he led. Although trained in the skills of a Watcher, his overwhelming fear initially prevented him from gaining experience in real-life combat, and rendered him useless in fights.
However, as his personality hardened, Wesley gained experience during his time with Angel and became an incredibly skilled martial artist, as well as being proficient with weapons. He was able to take on vampires with success and subdued Justine Cooper single-handedly on two occasions.
The best marksman with firearms in the Buffyverse, he is able to hit a tiny target while performing acrobatics (e.g. in episode "Inside Out", when he killed the demon Skip). His superior and detailed knowledge of demons and sorcery aided Angel and the team regularly.