In Mario Kart DS, the Boo item allows a racer to steal an item from another racer. When activated, Boo also makes the user temporarily invisible and protects them from many attacks and obstacles.
Boo typically targets a nearby opponent carrying an item, takes that item, and delivers it to the player who used Boo. This makes it especially valuable when a racer is trailing someone with a powerful defensive or offensive tool. The exact behavior differs from game to game, so descriptions of Boo from other Mario Kart titles should not automatically be applied to Mario Kart DS.
Boo is often confused with the Blooper because both are recognizable Mario characters used as race items. Blooper sprays ink across opponents' screens, obscuring their view, but it does not steal anything. Hammer is an offensive item in later Mario Kart games, while Ghost is a generic description rather than the item's official name. Boo first appeared as a Mario Kart item in Mario Kart 64.