Costume Kingdom

There are 11 Costumes available in the game but only 10 are required for this trophy. Most are going to be Story/Quest related while a few you pick up in the open world. Their locations are as follows:

• Turtairy: Story Related. This is the first Hallowmon/Costume you pick up during the Tutorial.
• Owly: Located in the first area of the game. From the starting house (with the large glowing green 'portal' in front of it), follow the dirt path left and it will be at the very end [of the path]. It'll be right next to Cat/Hallowmon #6 of the Maggies Madness quest as well so please see Maggies madness if you're still having trouble finding it.
• Tortoros: Earned after you complete Apex 1 (see The journey begin's).
• Crocross: Earned after you defeat Cranky Crocross in the Lake area (see Cranky Crocross).
• Bearibou: Earned after completing the Crocross Bounty-Board Quest (see Bounty - Crocross).
• Magicoon: Earned after completing the "And Your Small" Quest in the second area of the game (see And your small).
• Ramking: Earned after you complete Apex 2 (see The push).
• Chickolix: Located in the Cypher HQ. Just to the right (down the stairs) of the portal that leads to the third Apex will be a chest with the Costume/Hallowmon inside.
• Magmos: Earned after you complete Apex 3 (see Journey's end).
• Chimpeat: Earned after you complete the Marcus the Maniac Quest (see Marcus the maniac).

Stratus is the final Costume/Hallowmon you'll unlock but is not required for this trophy. None of the Hallowmon are missable and you can go back to the open world once you finished the game so after you've collected the last one, whichever one it may be, the trophy will unlock.