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A Boo Coin is an item in Puzzle & Dragons: Super Mario Bros. Edition that allows the player to transform certain types of enemies into stronger forms. Boo Coins can be obtained by defeating Boo-type enemies or by winning them in the Lucky ? Blocks game in Toad Houses.
Transforming enemies into stronger forms require a certain amount of Boo Coins. Transforming Boos into the enemy "Boo Trio" requires three Boo Coins, and can be upgraded even further to the enemy "Boo Crew" for six more. Whomps can also transform with Boo Coins into the enemy "Whomp & Boos" with six Boo Coins.