Amy Pemberton has been billed as a main-cast regular from the start, and so was Keiynan Lonsdale during his tenure. Just read the credits -- that's how you tell a regular (listed among the "Starring" cast, before the creator credit, or in the dedicated main title sequence if a show has one) from a recurring or guest player (listed among the "Guest Starring" credits). The difference isn't about how many episodes someone's in or how big a role they have, it's about whether they're under contract as a main cast member or not. So there have been a number of cases where a nominal recurring guest has had a larger amount of screen time than a nominal series regular. (For instance, "Special Guest Star" Martin Landau vs. nominal lead Steven Hill in the latter part of Mission: Impossible's first season.)
