At the opening of the novel, Severian saves the life of Vodalus, an outlaw who is fighting to restore Urth to its former glory. As a reward, Vodalus gives Severian a gold coin, which Severian treasures from then on. Severian later learns that the golden coin is fake. Dr Talos hands him a similar coin and tells Severian it is fake; Severian compares the coin to the coin Vodalus gave him, and notices they are the same (pg. 395, Citadel) Throughout the rest of his youth, Severian idolizes Vodalus and seeks to join him.
Severian starts the novel as an apprentice but shortly becomes the captain of apprentices, and later a Journeyman in the Guild of Torturers. While still captain, Severian meets and befriends the Chatelaine Thecla, a woman who is part of the Autarch's court who has been imprisoned because her sister had become the consort of Vodalus. Thecla is to be treated with special care, and since she takes a liking to Severian, asks that he sit with her for a while each day.
Severian eventually falls in love with Thecla. However, when Thecla is eventually tortured, Severian is moved by love and pity, and sneaks a knife into her cell, allowing her to take her own life and end her suffering.
Severian is briefly imprisoned, but it is decided that his life is to be spared, and (after being given the sword Terminus Est) he is sent to be the Lictor of Thrax, a small city in the north. He is also allowed to keep his Torturer's cloak, which is fuligin, "the color that is darker than black."
As he exits the Necropolis he must pass through the city of Nessus. As he does this, he meets Dorcas and Agia, two women who play a vital role in his life during the narration of the novel (see below). He also obtains the Claw of the Conciliator, a gem with apparently supernatural powers that Severian is attempting to return to its rightful owners, the Pelerines. During his travels, he eventually meets with Vodalus and agrees to join his insurrection. At a feast that evening, Severian ingests a portion of Thecla's preserved brain together with a drug taken from an (apparently alien) creature called an alzabo, a ritual that conveys to him Thecla's memories. Part of her personality and presence occasionally emerge and some characters seem to recognize, or sense, Thecla when they look at him.
When Severian reaches Thrax, he thrives as the city's Lictor for a while. However, he once again shows mercy (allowing a woman to escape rather than strangling her to death) and is forced again to flee.
Severian travels to the House Absolute, the throne of the commonwealth's Autarch, and eventually abandons Vodalus' cause. In additional travels, he eventually becomes a mercenary in the north and is seriously injured. During his convalescence he encounters the Ascian language before venturing out once again. Towards the end of the final volume of Book of the New Sun, he encounters the Autarch himself. When the Autarch is critically injured, Severian inherits the throne.
Severian apparently rules for quite some time before leaving Urth to seek a new sun, as some of his predecessors as Autarch had tried (and failed) to do; these events are covered in the follow-up novel "Urth of the New Sun," which is not considered an integral part of the initial 4-volume "Book of the New Sun."
Dorcas, whom Severian inadvertently resurrects at the Lake of Birds, is almost certainly Severian's paternal grandmother. (Wolfe has named her for the Biblical Dorcas, who was also resurrected.) The old man at the Lake of Birds and Dorcas had Ouen, the waiter at the Inn of Lost Loves, who refers to Dorcas as his "mother come again." Ouen later tells Severian that a locket he has contains a picture of said mother; Severian recognizes it as Dorcas. The innkeeper there then notes that while Ouen does resemble his mother's picture, in profile he's very like Severian.
Severian quizzes Ouen about his past loves, asking "A woman you loved--or perhaps only one who loved you--a dark woman--was taken once?" Ouen confirms that a woman named Catherine was taken by the law (and therefore handed to the Torturers) after having run off from some religious order (probably the Pelerines). Catherine's child was raised by the Guild, which is where we find the young Severian at the beginning of the book.
Despite being the narrator for all four books of the series plus a sequel, Severian's personality and inner world remain unknown, even to Severian himself.
At one point in his narrative, Severian refers to himself as insane, although he is not entirely certain how and why this is. As a professional torturer, despite his displays of mercy, he tends to be extremely clinical and detached when describing his work activities, to the point where he assumes the reader is already familiar with his methods and rationalizations.
By the middle of the first book, he carries with him, at all times, an unusual artifact, the "Claw," which apparently has the power to bring the dead back to life, as he does with Dorcas and, later on, a young soldier (before he brings this young man back to life, Severian seems to realize that he almost prefers the company of the dead to the living).
Severian describes himself as having a straight nose, deep-set eyes and sunken cheeks. Thecla states she has "never seen such white skin coupled with dark hair." Regarding Severian's appearance of strength, the Autarch remarks that Severian "seemed to me a construction of horn and boiled leather." He is said to be tall, although not at genetically-altered exultant levels. Cyriaca, who never saw him without his mask, describes Severian as having a narrow waist, a sharp chin with a cleft, deeply set, large and mobile eyes, high cheekbones, flat cheeks, black hair and thin lips.
He is usually dressed in the uniform of his guild: a fuligin mask, fuligin cloak and fuligin breeches, a belt, hose, black boots & a bare chest. He also carries his sword, Terminus Est, in a "sable manskin" sheath slung over his left shoulder in a baldric. He carries his few possessions in a sabretache attached to his belt.
Severian's height and apparent strength, along with his intelligent and educated manner of speaking, repeatedly lead him to be singled out for special treatment by people who have known him only briefly. These include Dr. Talos, who offers Severian a partnership; the Autarch, who recognizes him as his successor; Cyracia, who seduces him; the people of the lake, who follow him into battle; Folia, who asks him to judge a story contest; Mannea of the Pelerines, who sends him on an important mission; Guasacht, who offers him a place in the cavalry; and the captain of the Samru, who gives Severian free passage and protection on his vessel. Meeting Severian at the Saltus fair, the green man says, "I'm a fool, I suppose, to put any confidence in you. And yet I do." However, the Cumean's acolyte Merryn calls him "common," and none of the servitors or courtiers of the House Absolute pay him any heed, except for steward Odilo, who takes him for a nobleman [and later, the younger Odilo, in The Urth of the New Sun, who does the same.]