Lord Byron, according to his dumped mistress Lady Caroline Lamb, was “mad, bad and dangerous to know”. Antony Peattie’s exploration of his personal caprices and intellectual quirks definitively strikes down all three charges. Byron the self-aware ironist was never demented; he may have relished his reputation for vice, but his pagan promiscuity was overshadowed by the legacy of his punitive Calvinist upbringing; and it would surely have been a delight, not a danger, to know this convivial fellow, whose eyes, as Coleridge said, were “the open portals of the sun” and his teeth “so many stationary smiles”.

Peattie’s biography starts with an anecdote about Byron’s teenage years that encapsulates his slippery psychological complexity. On an evening of amateur theatricals, he performed first in a sulphurous melodrama, then in a comedy of manners. In one play, he was a misanthrope branded with the mark of Cain, in the other a frivolous dandy. “Everything by turns and nothing long”, as he said, he found both the outcast apostate and the man of mode inside himself. Or were they simply masks Byron wore and then discarded?

He invented Byronic heroes – Childe Harold the unspiritual pilgrim, the guilt-racked Manfred – to whom he attributed devilish sins he had not actually committed. As his last alter ego, he adopted the mythic figure of the libertine Don Juan, yet the fumbling, immature character in his comic epic hardly measures up to the defiantly irreligious seducer in Mozart’s Don Giovanni. As Peattie puts it, Byron’s Juan is as unlike the archetype as a stupid Faust or a stay-at-home Ulysses would be.

These contradictions expose a self-dissatisfaction or self-disgust that undercut Byron’s glamour and swagger. His club foot, he thought, made him damaged goods; he blamed this disfigurement on his mother, who wore a corset throughout her pregnancy and refused to remove it when she went into labour. He also had a tendency to obesity, for which his plump mother was likewise held accountable. His remedy was a succession of absurd, obsessive diets, including one that confined him to hard biscuits and soda water, supplemented by tobacco as an appetite suppressant.

Byron’s aversion to “animal food”, meaning meat, dramatised his alienation from England, since John Bull proverbially gorged on roast beef. “To be carnivorous,” Peattie says, “was to be patriotic”: remember Boris Johnson in Biarritz lecturing Trump on the exportability of British pork pies? But Byron’s abstinent regime so weakened him that his death in the Greek marshes was as much due to starvation as to the quackery of his doctor, who diagnosed epilepsy and siphoned off half the blood in his body.

Peattie probes the neuroses of the private man, but his book’s rich parade of illustrations also documents Byron’s self-presentation as a public figure. “I go forth,” says the hero of his poem Sardanapalus, “to be recognised.” When Byron met Ali Pasha, he exoticised himself in an Albanian costume, complete with sabre. Setting off for the Greek war of independence, he commissioned a plumed, crested helmet based on a Homeric prototype and posed with it wearing a tartan cloak that alluded to the battling heroes of Walter Scott’s novels. He died before joining the fight, but at least he had assembled the right accessories. Byron was always a fussy curator of his own image. A sketch made in Venice gives him unkempt whiskers, which he grew defensively when told that he had “the bloated face of a castrato”. A neoclassical bust made him look, he complained, like “a superannuated Jesuit”.

For his most ardent admirers, replicas were not enough: his fans undressed or unpeeled him. Peattie’s exhibits include a shirt Byron wore, now owned by a private collector of memorabilia; unfortunately, he can’t show us the scrap of Byron’s blistered skin that was kept as a memento by his current mistress after he suffered sunburn. Everyone wanted a piece of Byron and he was happy to give himself away. In Childe Harold’s Pilgrimage he asks: “What am I?” and promptly answers: “Nothing.”

Peattie’s book is the portrait of an elusive, paradoxical man, a poet who thought that words were as expendable as breath, a narcissist who disliked himself and a celebrity who laughed at his own publicity.

• The Private Life of Lord Byron by Antony Peattie is published by Unbound (£35). To order a copy go to guardianbookshop.com or call 0330 333 6846. Free UK p&p over £15, online orders only. Phone orders min p&p of £1.99