Mattheo Riddle isn’t a character that appears in any of the 'Harry Potter' films or the original book series by J.K. Rowling. The name might sound familiar because it echoes 'Tom Marvolo Riddle,' who, of course, is the birth name of Lord Voldemort. But Mattheo himself doesn’t exist in the official canon—no mentions in the movies, books, or even the extended universe like 'Fantastic Beasts.' I’ve seen some fan theories or creative AUs (alternate universes) where fans invent new characters, and Mattheo could be one of those, maybe a sibling or relative of Voldemort in someone’s headcanon. The idea of another Riddle lurking around is intriguing, though! It makes me wonder how the story might’ve changed if Voldemort had family ties that weren’t just the Gaunts or the Peverells.
If you stumbled across this name in fanfiction or a roleplay community, that’d make sense—there’s a ton of inventive lore fans have built over the years. But for official content, the Riddle lineage is pretty much just Tom, his dad, and his estranged wizarding family. Honestly, I’d love to see more exploration of the Riddle backstory in future adaptations. A spin-off about the Gaunts or young Tom at Hogwarts could be darkly fascinating. Until then, Mattheo’s staying in the realm of fan creations, which is where a lot of the magic of fandom thrives anyway!