Marnie is one of the player's rivals. She is a stoic, but well-mannered girl who wants to become Champion to support her hometown Spikemuth. Her first Pokémon is her Morpeko, who can be seen following her around outside of battle. She is constantly followed by her fan club, Team Yell. She can first be seen at Motostoke Stadium while the player and Hop register for the Gym Challenge. The player first meets Marnie in the lobby of Motostoke's Budew Drop Inn, where she scolds a group of Team Yell Grunts for causing trouble for the player, Hop, and other participants before introducing herself and apologizing on her behalf. After the player returns to Motostoke, the player has their first battle with her in the same lobby. The next day, Marnie will give them a copy of Kabu's League Card and two Burn Heals before seeing them off. The player later meets Marnie and Morpeko at Ballonlea Stadium, where she learns her Morpeko has taken a liking to the player, and she gives them a copy of her League Card before officially acknowledging them as a rival.
Later, on the outskirts of Spikemuth on Route 9, Marnie will show the player an alternate way into town but insist on a battle before they can enter. After being defeated, Marnie gives them Piers's League Card and leads them inside, claiming she'll get her revenge in the Champion Cup. During their Spikemuth Gym Mission, the player will learn that Team Yell consists of the Gym's Gym Trainers, and that they blocked the main entrance to cut down on Marnie's competition. They also learn that the Gym Leader, Piers, is Marnie's older brother. After Piers is defeated, Marnie arrives to congratulate both her brother and her rival on the match and tells Piers she knows he plans to retire his title after the Champion Cup. Piers planned on offering the position to her, but Marnie refuses, telling him she can't be a Gym Leader if she becomes Champion. The player leaves Spikemuth while Marnie and Piers have their Gym Battle.
The player next meets Marnie as their first opponent in the semi-finals of the Wyndon Stadium Champion Cup. Marnie says she was expecting them, and that despite what they've been through together, she declares she's going to go all-out. Marnie is revealed to have a Dynamax Band, unlike her brother, and uses it to Gigantamax the newest addition to her team, Grimmsnarl. After being defeated, Marnie thanks the player for the battle, saying that she enjoyed putting on a show for the crowd, but tells the player she hasn't yet decided who to root for just yet. After the semi-finals, the player and Hop meet Marnie, Piers, and Team Yell in front of the Rose of the Ronderlands hotel, who agree to help them look for Leon. While the player battles Macro Cosmos's Eric, Marnie will lead Team Yell in cheering for the player, boosting their Pokémon's stats. As the player and the others reach Rose Tower, Marnie and Piers will stay behind to hold off any other interlopers. Marnie later appears in the waiting room of Wyndon Stadium after Piers loses his match with Raihan in the Champion Cup finals, telling the player she'll be cheering for them.
After the player becomes Champion, they'll find Marnie in one of the rooms of the Budew Drop Inn, talking to her Morpeko. She attempts to practice smiling by pushing up the corners of her mouth, but when she is caught by the player, she runs off embarrassed. The player can then find her in Spikemuth, where she reveals she did take up Piers's offer, taking his place as Gym Leader. The player can train with her in a rematch here once daily. After being defeated for the first time, she will give the player a copy of her rare League Card. After obtaining this Card, they can encounter her in rematches in Champion tournaments at Wyndon Stadium, as well. Here, she will use the same team, only she will also Gigantamax her Grimmsnarl once more.
"As a child, she was timid and a bit of a crybaby. However, she became more cheerful as she played with the Pokémon she received from her brother, and her talents as a Trainer also grew greatly. She joined the Gym Challenge because she wishes to breathe life back into her dilapidated hometown, but she says she has been somewhat distracted by the boutiques throughout Galar. Though many in Spikemuth see her as a local celebrity, she herself has no interest in pursuing fame. However, she says she does appreciate the support of Team Yell, her personal cheering squad."
Rare League Card
"She was appointed as the new Gym Leader of Spikemuth by her brother Piers. Piers caught Morpeko and gave it to her when she was five. She still finds herself at the mercy of Morpeko's mood swings even now. The local dialect of Spikemuth has a distinct kick to it, and many of Marnie's fans have started mimicking her. Marnie won't change her expression even when she gets angry, but once something sets her off, she stays angry for a long time."
Marnie debuted in the Sword & Shield chapter. She was first seen in front of the Galar Mine, unable to enter due to a large boulder blocking the entrance. Marnie, Shirudomiria, and Hop combined their Pokémon's attacks to move the boulder. Later, Marnie watched Sōdo's Gym battle against Milo.
Morpeko is Marnie's only known Pokémon. It was first seen helping move a boulder blocking the way to Turffield.
Marnie was introduced as a Supporter card in the Pokémon Trading Card Game during the English Sword & Shield Series (the Japanese Sword & Shield Era). It was first released in the Japanese V Starter Sets. It later appeared in the Japanese Shield expansion before debuting in English in the Sword & Shield expansion, with artwork by kirisAki. It was also released as a Full Art card and Secret card in the same Japanese and English expansions, with artwork by the same artist. It allows both players to shuffle their hands to the bottom of their decks. If so, the card user draws five cards and their opponent draws four.
Marnie's uniform number is "960", which can be read in Japanese as goroawase for 苦労 kurō, meaning "hardship", or 黒 kuro, meaning "black".