The young man calling himself Aegon Targaryen was raised across the Free Cities under the name Young Griff, the supposed son of a disgraced Westerosi knight, and only revealed as a claimant to the Iron Throne once his tutors judged both the boy and the political weather ready. His backers' account holds that he is the true son of Prince Rhaegar Targaryen and Princess Elia Martell, smuggled out of King's Landing as an infant and swapped for a miller's son who died in his place during the Sack of 283 AC — a substitution the Citadel can neither confirm nor rule out, since the only witnesses who might settle it are either dead, loyal to the claim, or, in the case of the man who arranged it, famously difficult to pin down on anything.
He was raised and trained for exactly this return by Jon Connington, a Kingsguard-adjacent exile once close to the real Rhaegar, posing throughout as the boy's father under the name Griff; a scholar called Haldon, called the Halfmaester for training he never finished at the Citadel, saw to his education. The result, by every account of those who've met him, is a young man conspicuously well-suited to the role he's about to play — martial, bookish, and gracious in something close to equal measure, which is either exactly what a Targaryen prince raised with unusual care would look like, or exactly what a coached impostor would be drilled to resemble.
He has landed in the Stormlands with the Golden Company at his back, taken Griffin's Roost and pressed toward Storm's End, and the Iron Throne he means to claim has not yet had cause to take his claim seriously — though men who ought to know better, Tyrion Lannister and Ser Barristan Selmy among them, have both voiced open doubt of his identity without being able to disprove it outright. The Citadel records the claim as contested and the boy's true parentage as, for now, genuinely unresolved.