Xavier Riddle is the eponymous character of the series, Xavier Riddle and the Secret Museum, being one of three main human characters: himself, his sister Yadina and their friend Brad.
Xavier is a young boy with tan skin and shiny black hair. He wears grey-blue pants and black sneakers over white socks.
He wears a red hoodie with the hood up, "alien"-style antennae on it, and a black "X". This iconic hoodie contains The Translator which allows him (and it seems, also Yadina and Brad, despite not wearing one too) to understand non-English speaking historical figures.
Xavier is the leader of his group with Yadina and Brad. He appears to be an excitable kid who is eager to go back in time and meet various historical figures. He is described as a kid who loves everything (especially art and basketball), but lives for adventures.
In concept art, he was originally white, had brown hair, black pants, a red sweater, and lightning bolts instead of antennae.
The "X" on his hoodie stands for Xavier.
As of November 2, 2019, he has a different voice on the show's official website. The reason for this is unknown, though in the case of characters voiced by children this usually occurs when the character's voice actor become old enough that their voice deepens.