The downside of always have Ash as lead is that no other player characters get a fair shot, this is why Pokemon Masters exists.

Rosa is the female player character in Black and White 2, probably one of if not the best female design of the Pokemon games, the Pokestar studios made her an actress in the canon playing everything from a princess to a super hero, so for her to be picked as the lead for masters, the creators wasted no time in giving her an endearing personality, instantly cheerful, full of energy and her now memorable thinking pose; she even made the cover of the game, this is alongside some high level alumni including Steven Stone, Diantha, Lance, Cynthia, Red and Brock, five of which are champions while Brock has the legacy of being the first ever Gym Leader and because different outfits are a thing, have her Christmas dress.