Marnie

Marnie is a rival in Pokémon Sword and Shield. She is the younger sister of Piers, whose position as gym leader she later takes. Piers created Team Yell to support Marnie, but the group got out of hand, and started harassing other trainers, despite the wishes of Marnie and Piers.

Trivia

 * Marnie and Piers are the first gym leaders to specialize in the Dark-type.