An aristocratic, he uses his connections in high society to assist the efforts of the Conspiracy, largely to protect the interests of the gentry from Hiram Burrows influence. He is the younger brother of the twin lords Custis and Morgan Pendleton. Although his affiliation with Corvo Attano it's later revealed that he plans on betraying Corvo, along with Farley Havelock and Teague Martin.
The third and youngest child of the late Lord Pendleton, little is known about Treavor's parents or his relationship with them. In his memoirs, he notes that he had a "despised stepmother" during his youth who left the family when he was thirteen, which caused his father to sink into a deep depression[; it is suspected that Lord Pendleton perished soon after, leaving the family fortune to Custis and Morgan.
Treavor endured cruel and extensive torment at the hands of his older brothers throughout his youth. For instance, he explains that at five he was tied to his crib and left to the mercy of a "bundle of vipers" that they placed next to him, which nearly led to his death. His brothers' cruel treatment continued into his adult life, with Treavor stating that he was nearly killed by his brothers during a hunting trip immediately prior to the events of Dishonored.
During Treavor's teenage years, the Pendleton clan maintained a familiar relationship with the Boyle family. Treavor met Waverly Boyle when he was thirteen and may have had feelings for her, which he notes in his memoirs, saying she is "[the] source of many tender recollections [who] came into our cold, marble halls and brought light and warmth." At some indeterminate time, the friendship dissolved; Treavor claims that "It was only later I realized she was a traitorous little weasel," though what brought his change of heart is not specified. Despite the hostility with which he regards her, his frequent references to her suggest that this affection may not have entirely dissipated.
From his youth through adulthood, Treavor appears to have maintained consistently poor health; he mentions that the viper incident "kept [him] convalescing for months," and the Heart notes that he is prone to skin infections and suffers from light sensitivity. He also has a drinking problem, which is exacerbated by his nervousness and guilt concerning the Conspiracy—many liquor bottles can be found under his bed at the Hound Pits Pub, and his manservant, Wallace Higgins, laments not being able to find a "proper vintage" for him.
During his time as a Loyalist, Treavor stays at the Hound Pits Pub with Wallace, although he does occasionally leave to travel to Pendleton Manor.
After aligning himself with the Loyalist Conspiracy, Pendleton seeks control of his elder brothers' Parliamentary holdings. Before the House of Pleasure mission, Pendleton asks Corvo to kill both of his brothers so that he may use their Parliamentary votes to aid the Loyalists' aims. In a letter found in his quarters before the mission, Pendleton reveals that his main goal is to stop legislation that would put the holdings of Dunwall's gentry at risk.
At first, Treavor is evidently hurt by his brothers' deaths. When Corvo returns from the Golden Cat, Pendleton behaves in a highly antagonistic, passive-aggressive manner toward him. However, if Corvo aids Slackjaw during the mission (which results in the Pendleton brothers being kidnapped and forced to work in their own slave labor mines), Treavor discovers that they are alive through his contacts. In this case, he will leave a thank-you letter, as well as two gold ingots, in Corvo's quarters as a reward. He expresses that it does not matter to him that they are miserable, so long as they are alive, so that one day he may see them again.
He is later found to have been conspiring with Farley Havelock and Teague Martin to eliminate Corvo and the other members of the Loyalist Conspiracy to cover up their crimes. After poisoning Corvo, the three betray the other members of the Conspiracy, kidnapping Emily and fleeing to Kingsparrow Island.
Whether Pendleton is one of masterminds of this plan or simply went along with Havelock and Martin is unknown; Callista Curnow suggests the latter when Corvo returns to the Hound Pits Pub during The Loyalists mission, and Anton Sokolov expresses similar sentiments.
Pendleton's ultimate fate changes based on Corvo's chaos level.
In low chaos, Treavor is found dead at a large table along with Martin during Corvo's final mission, presumably poisoned by Havelock.
In high chaos, Treavor is discovered barricaded in the Kingsparrow Gatehouse in the midst of a conflict with Martin, who cannot pass into the lighthouse due to Pendleton's barricade. In the midst of their argument, Martin fires at Pendleton, then leaves to regroup and plot how to get past the gates.
Pendleton retreats behind a destroyed wall and can be seen telling the captain of his personal guard that he is dying. When Corvo approaches, he will see Pendleton holding his bloodied side. It is unclear who actually shot Pendleton; he tells his captain "That bastard's done me in," referring to Martin, but tells Corvo he was taken by a stray bullet and will "never know whose."
If Corvo does not immediately kill Pendleton, the man will mock Corvo, saying, "everyone knows you were screwing the Empress." He will also advise that Corvo seek out Pendleton's cousin Celia if he "like[s] noblewomen."