After finding Kabu in Galar Mine No. 2, you’ll need to return to Motostoke and stay the night in the Budew Drop Inn. Whether you choose to explore the optional Motostoke Outskirts is entirely up to you, though.
When you visit the Budew Drop Inn to the west of the Motostoke Stadium, you’ll have an encounter with Marnie and she’ll try to get you to battle her. You have a chance to decline if you need to get prepared, but otherwise get ready for a battle against her. Keep in mind that your Pokémon will not be healed before the fight, so you might want to heal at the Pokémon Center first if they’re weakened from your recent trip.
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Marnie primarily uses Dark-type Pokémon, but also has a Croagunk that should counter any Fairy- or Fighting-types looking to take advantage of that weakness. Her Croagunk also has Sucker Punch to counter any Psychic-type Pokémon, so be cautious of that.
The Morpeko on her team is probably the biggest threat on her team and hits surprisingly hard. You’ll see it change between its Full Belly and Hangry Modes between each turn, but its type remains the same regardless of which form it is in—Dark/Electric. It only affects its signature move, Aura Wheel, which it does not have in this fight and won’t get until much later, so don’t worry too much about seeing it change its form.
After you beat her, you’ll stay the night at the inn and will meet up with Marnie the next morning. She’ll give you Kabu’s League Card, 2 Burn Heals, and tell you that Hop already went off to the Stadium.
Head outside and then return to the Motostoke Stadium. You’ll run into Hop there, but don’t worry, you won’t have to battle him. He’ll wish you luck and will then let you enter the Stadium to take on Kabu and earn your third Gym Badge!