Boo

From Mariopedia

Boos are enemies in the Mario series. They resemble the stereotypical ghost in their most common form. In the majority of games that they feature in, they appear to be shy, and stay still when looked at by the player, but advance in order to attack turned to them. Among ordinary varieties of Boos, there are notable individuals such as Big Boo, King Boo, Boolossus and The Speedy Spookster.

In Super Mario World and some games in the Super Mario Bros. series, they are found in most of the ghost houses, where they cannot be defeated by normal means and harm Mario if touched.

In Super Mario 64, Boos are found in the Castle Courtyard, where they hover near the entrance to Big Boo's Haunt. They release a blue coin when defeated, which can only be done when they are solid: they render themselves invisible and invulnerable when directly looked at. Super Mario 64 DS has Big Boo as a boss in Big Boo Battle as well.

Luigi's Mansion has Boos kidnapping Mario, the final boss in the game being King Boo.

In the Mario Party Series Boo appears as a playable character in Mario Party 7 and 8 also in the Mario Kart Series as a heavyweight character as King Boo.

In Paper Mario: The Thousand-Year Door, in Doopliss's palace, many boos are seen. Though not enemies, if certain requirements are done, all the Boos will fuse to form Atomic Boo, who is a sub-boss.