Marnie is one of the rival characters in Pokemon Sword and Shield, the others being Hop and Bede, and specializes in Dark-type Pokemon. She eventually replaces her older brother, Piers as the Gym Leader of her hometown Spikemuth. As the Spikemuth Gym Leader, she gives the Dark Badge to Trainers who defeat her.
In the Legacy of Chris Thorndyke, as she replaces her older brother Piers as being the Gym Leader of Spikemuth, she also became the new leader of Team Yell, telling the team that it was time to have the young blood lead to victory.
And she is voiced by Laura Bailey in the English dub and her Japanese voice is unknown.