Boo

Boos (, Teresa) are enemies from the series.

Origin
Boos made their debut in  as a common enemy, and has become a common enemy in later Mario games. They are white, spherical ghosts with short stubby arms and a short stubby tail. They have beady black eyes, a fanged mouth, and a large tongue they stick out occasionally.

Boos are known for appearing in s and other haunted levels in the Mario series. When the player is facing away, they will pursue the player and damage them on contact. When the player is facing them, they stop their pursuit, cover their bashful faces, and becomes intangible. In any case, they cannot be defeated under normal circumstances (e.g. jumping on them); however, they can be defeated by running into them under the effect of the Super Star or throwing a at them. In later Mario games, they can disappear momentarily when hit by fireballs or 's tail whips.

Boos have become a recurring item in the  series. When used, it will turn the player temporary invisible while the Boo steals an item from another player. If there is no item to steal, however, it will return with a Super Mushroom.

Prominent members of the Boo species include, the king of the Boos and the main antagonist of the ' series; and , a female Boo and one of Mario's party members in '.

As a background character
Boos appear as background characters in the Luigi's Mansion stage, only appearing briefly from the rubble of the mansion when part of it is destroyed.

Sticker
The Boo sticker can be obtained as a challenge reward by hitting Sandbag 900m/300ft in the Home-Run Contest.

As a background character
Boos return as background characters in the returning Luigi's Mansion stage in.

In Smash Tour
Boo appears as a red trophy item. When used, it allows the fighter to start the battle invisible.

Trophy
The Boo trophy appears in both versions. In, it is part of the Super Mario Bros. 2 & 3.

As a background character
Boos return as background characters in the returning Luigi's Mansion stage in.

Spirit
Boo also appears as a Novice-class primary spirit.

In Adventure Mode: World of Light, Boo is presumably one of the countless spirits captured by Galeem during his takeover of the universe. After Galeem's defeat, Dharkon takes control of Galeem's spirits, including Boo. Boo's spirit is located in the Dracula's Castle sub-dungeon of The Dark Realm.

Boo's Spirit Battle uses a puppet fighter and is fought on the Mushroom Kingdom U stage's Slide Tower layout. During the battle, Kirby frequently shields and turns invisible, referencing how Boos cannot be defeated under normal circumstances and turns invisible when faced by the player.