Marked Pokémon are a special type of Pokémon that can only spawn in the wild. If you catch a Marked Pokémon, you won’t even know until you check the menu under the Ribbons section. However, they are extremely rare, and there are plenty of players out there who have never seen them at all. They will only appear in the wild, as overworld, random grass, or fishing encounters.
Marks will convey a title on the Pokémon, and the title will vary, depending on the mark. The nickname will appear as a special tag line when you use the Pokémon in battle.
The Isle of Armor DLC introduced a new item called the Mark Charm. You can get the Mark Charm by finishing your Isle of Armor Pokedex and returning to the lady at the translation who gave it to you at the start of the campaign. When you show here the full Pokedex, she will give you a Mark Charm as a reward. This will give you a small increase to your chance of meeting Mark Pokémon in the wild.
