Boo

Boos were introduced in Super Mario Bros. 3 as an enemy, and since then have appeared as a playable character in many games. There have been many variations of Boo introduced throughout Mario's adventures. These include Big Boo, King Boo, Boolossus and numerous others. They are a floating, ghost-like creature.

Appearances
Boos first appeared in Super Mario Bros. 3, mostly in castles and fortresses. They won't move if Mario/Luigi faces them, but they'll give a chase when they turn their backs on them. In this game they were known as Boo Diddleys (perhaps a nod to Bo Diddley).

Boos were given extended play in Super Mario World, which featured hundreds of them residing in various "ghost houses", with the ability to disappear, group together, rotate in impenetrable circles and make various faces. When Mario or Luigi look toward these creatures, they blush, flinch and stop moving. When the characters turn away, the ghost gives chase. In one level, there is a variant that turns to stone — stone which Mario can safely jump on — when Mario faces him. Eeries (Telesauls in Japan) appear to be a dinosaur version of Boos, but are not shy. Instead, they wander back and forth in groups. In Yoshi's Island, Yoshi cannot hit Boos with eggs unless he is facing away and the egg ricochets off of a wall while Boos are active.

Though Boos are, for the most part, minor antagonists, one is Mario's ally in Paper Mario; Lady Bow, who assists Mario in his battle against Bowser, by making him invisible or scaring enemies.

In Super Mario Brothers Deluxe, a secret set of levels could be accessed if a certain number of points were obtained. In these levels (called "You vs. Boo"), Mario/Luigi raced Boo to the end of the level. Boo acted like a literal ghost in You vs. Boo, finishing at your time (and changing in color) every time you challenge him again after beating him.

In Luigi's Mansion, King Boo is the main antagonist in Luigi's quest to rescue Mario. (He reappears in Super Mario Sunshine; Super Mario 64 DS, oddly named "Big Boo" like the non-crowned large ghost of that game, except he called himself "the" Big Boo; is playable in Mario Kart: Double Dash!!, and appears in Mario Party 8'' as an NPC). Most Boos are shy but mischievous ghosts that love scaring others (especially Toads, as seen in Luigi's Mansion). Despite the fact that they are "ghosts", they do not seem to be the spirits of the deceased, but rather a separate type of creature.

Super Princess Peach introduced a reversal of this with the Mad Boo, a Boo that chased Peach whenever she looked at it, and cowered when she turned away. A larger boss version exists, known as "Big Boo" (who made its debut in Super Mario World) and a boss in Yoshi's Island is named "Bigger Boo." Other types include "Fishin' Boo" (Spook in Japan), who floats overhead dangling a lethal blue flame, and a Pink Boo from Super Mario Sunshine who, when sprayed by FLUDD, would turn into a platform for a short time.

New Super Mario Bros. features a Boo (called the Balloon Boo) which inflates itself to massive proprtions when stared at, and when Mario or Luigi looks away, it expels the air, propelling itself forward. This game also features a Boo-like creature called a Broozer, a pale ghost with green eyes, a sinister, fanged smile and boxing gloves, which could punch through solid blocks. The Yoshi series of games introduced Boo Balloon, who is a balloon version of a Boo who does not freeze if looked at directly, and Boo Man Bluff (an obvious reference to "blind man's bluff"), a large orange blindfolded Boo, who does not freeze if looked at directly. In Yoshi's Story, a blind variation exists, which approaches on basis of sound rather than sight, as well as a Slime Boo. Boo Guys are the ghosts of Shy Guys that appear in Super Mario World 2: Yoshi's Island, and Mario & Luigi: Partners in Time. Two variations known as "Greaper" and "Glum Reaper" appear in Super Mario RPG: Legend of the Seven Stars.

In Mario & Luigi: Partners in Time, Boos appear in Bowser's Castle. There, they attack simply, but they do not shy away from Mario and Luigi like in the platformers.

In Super Paper Mario, Boos haunt certain chambers in Merlee's Mansion, and Dark Boos inhabit parts of "The Underwhere", a realm where the dead spirits of sinners reside. This may lend evidence to Boos being the spirits of the dead, but it is also possible that they are merely a form of demon.

Boos also appear briefly in The Legend of Zelda: Link's Awakening, haunting a single room in Bottle Grotto. These Boos seem to have gotten over their fear of facing opponents and float carelessly toward intruders. Many of the game's weapons are useless against them, as they disappear when struck by them and appear on the diametrically opposite side of the room. When light touches them, however, they turn into retreating shadows and are vulnerable to nearly everything. The Magic Rod remains the only item able to dispatch their ghost forms.

In Mario's Picross, Boo appears as one of the puzzle solutions.

Mario Party
Boos were commonly featured in the Mario Party series. They would have their own space where players could steal each other's coins and stars. In some Mario Party games, Big Boo made an appearance with the role of a regular Boo but was able to steal coins and stars from all players. Starting with Mario Party 5, it became a playable character. Replacing it in its job would be Chain Chomp, and later, a pink Boo.

A Pink Boo appeared in Mario Party 6. When a player landed on its space, it would steal coins or a star from chosen player for up to 40 coins. The Pink Boo wasn't a space in Mario Party 7, but was an orb that stole stars/coins and gave it to the player; it also appeared in mini-games like Mathemortician and Ghost in the Hall.

In Mario Party 4, Pink Boo appeared on Boo's board and when passed, he will cause the pink bridges to appear/disappear. When visible, in game events such as the Ghost Train will be unusable due to their presence.

Boo is also a playable character in Mario Party 8.

Mario sports
Some Mario Kart games feature Boo as an item that makes the user invisible and steals an item from a rival. In Mario Golf: Toadstool Tour, Boo is a playable character that must be unlocked by getting 50 best badges in tournaments. In Mario Tennis and Mario Power Tennis a Boo is a playable character; he possesses the ability to curve the ball a great distance and thus confuse opponents.

Boo is also a playable character in Mario Hoops 3 on 3, and, Mario Superstar Baseball, along with King Boo. Boo is an above average hitter with poor defense and speed, but is a surprisingly good pitcher.

Boo will also be a playable character in the game Mario Strikers Charged, where he is a Playmaker sidekick.

Note that the enemy called Teresa in Japan has been translated with several names in English translations. The following are not variations: Boo Diddly in Super Mario Bros. 3 and The Big Boo in Super Mario RPG: Legend of the Seven Stars.

Trivia

 * Boo's laugh is a sped-up version of Bowser's laugh.