Marnie

You first encounter Marnie in Motostoke when you go to register for the Gym Challenge. As you go to register, Team Yell will get in the way and cause hassle. After you efeat them, Marnie will appear and apologise for their acts.

Much later, when you return to Motostoke to challenge Kabu's Gym, you'll encounter Marnie at the Budew Drop Inn. She will ask for a battle to see if she's ready for the next leg of her Challenge. Defeat her and she'll give you Kabu's League Card

Pokémon Trainer Marnie	Croagunk	Scraggy	Morpeko
Battle Type Items	Level 24	Level 24	Level 26
	Attacks: Poison Sting Venoshock Revenge Sucker Punch	Attacks: Beat Up Headbutt Low Kick	Attacks: Bite Quick Attack Thunder Shock
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item

When you reach Ballonlea Gym, you'll bump into Marnie again. She will mention how she has to watch out for you as you are a strong trainer and claims that her Morpeko is wary of you, however it turns out that the Morpeko actually likes you. She will give you her League Card and leave.

When you reach Spikemuth, you will see that the path in is closed, causing many Gym Challengers to get angry. Marnie appears and informs you of a shortcut into the city as she was born there. However, if you wish to get into the town, she insists on battling you

Pokémon Trainer Marnie	Liepard	Toxicroak	Scrafty	Morpeko
Battle Type Items	Level 42	Level 43	Level 43	Level 44
	Attacks: Fake Out Torment Nasty Plot Sucker Punch	Attacks: Venoshock Sucker Punch Swagger Poison Jab	Attacks: Brick Break Scary Face Crunch Swagger	Attacks: Quick Attack Torment Spark Bite
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item

Defeat her and she'll give you the League Card for Piers, the Gym Leader of Spikemuth and then say she'll go investigate why the shutters into the town were closed.

When you get through Spikemuth on your Gym mission, you'll learn Team Yell blocked off the town so there would be less challengers for Marnie. When you defeat Piers, you'll learn that Marnie is the sister of Piers, the head of Team Yell. He mentions that he hopes that Marnie takes over the Gym from him. Marnie declines as she intends to be champion and so it doesn't make sense as she'd be higher than a Gym Leader

The next encounter with Marnie is at the Pokémon League during the semi finals. She will be your first opponent and mentions how she knew you'd get all the badges and meet her there. She insists that despite what Team Yell did, she just wants to be Champion. She Gigantamaxes her Grimmsnarl

Pokémon Trainer Marnie	Liepard	Toxicroak	Scrafty	Morpeko	Grimmsnarl
Battle Type Items	Level 47	Level 47	Level 47	Level 48	Level 49
	Attacks: Fake Out Torment Nasty Plot Snarl	Attacks: Venoshock Sucker Punch Swagger Toxic	Attacks: Swagger Scary Face Brick Break Crunch	Attacks: Bullet Seed Torment Spark Bite	Attacks: Spirit Break Darkest Lariat Bulk Up Torment
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item
					Gigantamax

After the Semifinals finish, when Leon is late for dinner with you and Hop, Piers mentions Leon went off to Rose Tower and Marnie appears and is called in to help to cause a ruckus. However, when Oleana prevents everyone from seeing Rose and Leon, Marnie joins in and helps track down the League Staff to get in to Rose Tower.

As you're progressing through the finals later, Marnie will appear in the locker room with Piers and mention about how Piers ended up losing to Raihan. She is angry because people will think they're both weak due to how he lost. She then mentions how she's counting on you to win.

After completion of the league, you can find Marnie in the Budew Drop Inn in one of the rooms. When you enter, she will be speaking to her Morpeko about how so many people were cheering for her and so she wants to give them a proper smile, so is practicing smiling. When you come in, she runs off saying it's very embarrassing.

After completion of the league, you can find Marnie in Spikemuth. She will offer for you to have an exhibition match as she is now the Gym Leader. She insists it's her using you for training. This can be redone daily

Pokémon Trainer Marnie	Liepard	Toxicroak	Scrafty	Morpeko	Grimmsnarl
Battle Type Items	Level 59	Level 59	Level 59	Level 60	Level 60
	Attacks: Fake Out Torment Nasty Plot Snarl	Attacks: Drain Punch Gunk Shot Earthquake Poison Jab	Attacks: Ice Punch Thunder Punch Drain Punch Crunch	Attacks: Aura Wheel Seed Bomb Torment Quick Attack	Attacks: Spirit Break Darkest Lariat Bulk Up Play Rough
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item

When you compete in the Champion Cup again, Marnie is one of the trainers you can invite into the league or who will appear as standard throughout it. She once again Gigantamaxes her Grimmsnarl

Gym Leader Marnie	Liepard	Toxicroak	Scrafty	Morpeko	Grimmsnarl
Battle Type Items	Level 59	Level 59	Level 59	Level 60	Level 60
	Attacks: Fake Out Torment Nasty Plot Snarl	Attacks: Drain Punch Gunk Shot Earthquake Poison Jab	Attacks: Ice Punch Thunder Punch Drain Punch Crunch	Attacks: Aura Wheel Seed Bomb Torment Quick Attack	Attacks: Spirit Break Darkest Lariat Bulk Up Play Rough
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item
					Gigantamax

After completing the Isle of Armor and Crown Tundra, you will get invited by Leon to participate in the Galarian Star Tournament. There, Marnie will be there as a participant and will join you either as a partner or as an opponent

Gym Leader Marnie	Morpeko	Scrafty	Grimmsnarl
Battle Type Items	Level 73	Level 72	Level 74
	Attacks: Aura Wheel Seed Bomb Torment Quick Attack	Attacks: Ice Punch Thunder Punch Drain Punch Crunch	Attacks: Spirit Break Darkest Lariat Bulk Up Play Rough
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item
			Gigantamax

After completing the Galarian Star Tournament, then if you battle Marnie again she will now have a stronger team

Gym Leader Marnie	Liepard	Toxicroak	Scrafty	Morpeko	Grimmsnarl
Battle Type Items	Level 73	Level 72	Level 72	Level 73	Level 74
	Attacks: Fake Out Torment Nasty Plot Snarl	Attacks: Drain Punch Gunk Shot Earthquake Poison Jab	Attacks: Ice Punch Thunder Punch Drain Punch Crunch	Attacks: Aura Wheel Seed Bomb Torment Quick Attack	Attacks: Spirit Break Darkest Lariat Bulk Up Play Rough
	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item	Hold Item: No Item
					Gigantamax