Great-Uncle Algie is a member of the pure-blood Longbottom family and the great-uncle of Neville Longbottom. Though he never appears directly in the series, he is a significant figure in Neville's backstory. His primary role in the narrative is to illustrate the immense pressure placed on a young Neville Longbottom to prove his magical abilities and avoid the shame of being a Squib. Algie's dangerous and reckless attempts to “frighten” magic out of Neville ultimately lead to the first manifestation of Neville's powers and the gift of his pet toad, Trevor.
Algie Longbottom is a close relative of Augusta Longbottom, Neville's grandmother, though their exact relationship is not specified. He, along with the rest of the family, was deeply concerned that Neville might be a Squib due to his late development of magical talent. Driven by this fear, Great-Uncle Algie took extreme measures to try and force a magical reaction from Neville. On one occasion, he pushed a young Neville off the end of Blackpool pier; Neville nearly drowned, much to Algie's disappointment as no magic was produced. His most notable attempt occurred when he was visiting Neville's family. He dangled Neville by his ankles from an upstairs window, but became distracted when Great Auntie Enid offered him a meringue and accidentally let go. Rather than falling to his death, Neville bounced all the way down the garden and into the road. This incident was the first clear sign of Neville's magic. Overjoyed, Great-Uncle Algie bought Neville a toad, Trevor, as a celebratory gift. Years later, following the Battle of the Department of Mysteries, Augusta Longbottom told Harry Potter at St. Mungo's Hospital for Magical Maladies and Injuries that Great-Uncle Algie was “full of praise” for Neville's bravery and his part in fighting against Lord Voldemort's Death Eaters. This indicates a shift in his perception of Neville, from a source of familial anxiety to a source of pride.
Great-Uncle Algie's physical appearance is never described in the novels. Based on his actions, Algie can be seen as a reckless and somewhat desperate individual, willing to put his great-nephew in mortal danger for what he perceived as the greater good of upholding family honour. Despite the hazardous nature of his methods, they appear to stem from a well-intentioned, if misguided, desire for Neville to be a part of the wizarding world. He also appears to be easily distracted, as evidenced by his dropping Neville for a meringue. His immediate delight and generosity upon seeing Neville's magic, and his later pride in Neville's courage, show a more caring and supportive side to his character.
As a member of a respected pure-blood family, it is presumed that Algie is a competent wizard. However, no specific magical abilities, skills, or spells are ever attributed to him in the books.
The name Algie is a diminutive of Algernon. Of Old French origin, Algernon means “with moustaches” or “whiskered.” The name carries aristocratic and old-fashioned connotations in Britain, famously used for a character in Oscar Wilde's play The Importance of Being Earnest, which aligns with Algie's status as a member of an old pure-blood wizarding family.