Siegfried Sassoon – The fictional Siegfried Sassoon is closely based on the real Sassoon. Many reviewers of the novel describe Sassoon as the main character. Abandoned by his father as a child, the novel presents Rivers as a father-figure for Sassoon, which reflects their historical relationship. Despite Sassoon's decorated military career, his experiences in World War I caused him to publish an anti-war declaration. Although the character in Regeneration eventually returns to the front (as did the historical Sassoon), Barker depicts him as remaining deeply ambivalent about warfare. Moreover, Sassoon held ambiguous feelings about his sexuality throughout his life: though he married Hester Gatty in 1933, he had several homosexual affairs after the war.
Dr. W.H.R. Rivers – Based upon the real-life W. H. R. Rivers, Rivers is an English anthropologist, neurologist, and psychiatrist who worked at Craiglockhart War Hospital between 1916 and 1917, his patients included Siegfried Sassoon amongst other literary figures. Barker describes him as the main character of the novel (though some critics emphasise Rivers or Sassoon). Historically, he experimented with treatments for nerve regeneration with Henry Head. This research inspired the title of the novel as well as some of the trilogy's major themes, such as trauma, injury, and healing. In Barker's portrayal, Rivers suffers throughout the novel from the moral dilemma that he is treating soldiers in order that they can return to war. His approach is contrasted with the harsh treatment used by Dr. Lewis Yealland. Moreover, throughout the novel Rivers is struggling with a nervous stammer he has had since childhood, even though his own father used to be a speech therapist. In an interview with journalist Wera Reusch Barker called the historical Rivers "very humane, a very compassionate person who was tormented really by the suffering he saw, and very sceptical about the war, but at the same time he didn't feel he could go the whole way and say no, stop."
Billy Prior – Prior is one of the few purely fictional characters in the book. Prior is a soldier at Craiglockhart who suffers from mutism and asthma. According to critic Patricia Johnson, Prior's inability to speak highlights the novel's treatment of Western culture's inability to verbalise the mutilation of bodies caused by war. Prior is a working-class officer who has risen to the rank of lieutenant despite his background. Straddling the class divide, Prior sees the British army mirroring the class system, even in the trenches. Prior often envies those who are not involved in the war experience, such as Sarah, his love interest in the novel. As he develops in the Regeneration Trilogy, the novels reveal Prior as bisexual. He is a man fundamentally at war with himself: torn between his working-class roots and his army career, between his officially acknowledged love for Sarah and his "forbidden" sexual attraction towards other men, between his violent father and his fussing mother, his longing for peace and his hatred of civilians unaffected by the horrors of trench warfare.
David Burns – David Burns, another patient at Craiglockhart War Hospital, is a fictionalised version of one of Rivers' real patients who is described in the psychologist's case studies. Burns has been unable to eat after a bomb explosion threw him headlong into the gas-filled belly of a corpse, which caused him to swallow some of the rotting flesh. Critic Patricia Johnson explains that this experience of traumatic embodied experiences, epitomises the novel's strong use of visual descriptions of the war to help the reader recognise wars horrors (see the War themes section below).
Wilfred Owen – The fictional Owen is based upon the actual poet who died just before the end of the war in 1918. His posthumously published poems greatly increased his reputation. He is largely a peripheral character in the novel. Barker depicts Owen as initially unsure of the standard of his own poetry and asks Sassoon to help him revise them. These unrevised versions of the poems are not drafts originally by Owen, but rather versions of the poems revised by Barker. Owen's sexuality is also questioned, as Sassoon comments that Owen's feelings towards him seem to extend further than mere hero-worship.
Anderson – Anderson is another patient at Craiglockhart War hospital. Once a surgeon, Anderson's experiences of war have made it impossible to continue practising medicine because he now hates the sight of blood after experiencing a mental breakdown.
Sarah Lumb – Sarah is a completely fictional character. The girlfriend of the character Billy Prior, she is working-class, "Geordie," and works in a munitions factory in Scotland producing armaments for British soldiers. Ada Lumb, her mother, appears briefly and has a hardened attitude towards love and relationships.
Dr. Lewis Yealland – A foil to Rivers, Yealland is based on a doctor of that name at the National Hospital in London who used electro-shock therapy to treat his patients. Yealland is portrayed as arrogant and uncaring. He believes that the characters that breakdown during the war are "weak" and says that they would break down in civilian life anyway.
Callan – Callan is a patient of Dr. Yealland who has served in every major battle in World War I. He finds himself in the care of Dr. Yealland after suffering from mutism. Callan tries to fight against his doctor's treatment but eventually gives in to it.
Robert Graves – Another real life character, Graves is a fellow poet and friend of Sassoon who sees the war as unjust and immoral. However, Graves does not want to make his life more difficult by protesting. Graves sees it as his duty to serve his country regardless of his own moral beliefs.