Boos are recurring enemies in the Mario series. They are small ghosts that were introduced in Super Mario Bros. 3's. In their first appearance, they appeared as fortress enemies, and since then have shown as a playable character in many Mario games. There have been many variations of Boos introduced throughout Mario's adventures. These include King Boo, Pink Boo, Boolossus, Bomb Boo and Big Boo.
Super Mario Bros. 3
Super Mario Bros. 3 marks the first appearance of Boos. Boos appeared as timid and would cover themselves up if Mario or Luigi looked at them, but would chase them when they were turned away. The enemy were generally invincible unless a Hammer Bro. Suit was equipped, or there was use of a Blue Koopa Shell or Starman. Also appearing in Super Mario Bros. 3, are Stretches, a longer version of Boos that appear in Fortresses throughout the Mushroom Kingdom.
Super Mario World
Boos appeared in Ghost Houses in Super Mario World. Their attack pattern is the same as those from Super Mario Bros. 3 and will occasionally make faces at Mario and Luigi. A new sub-species of Boos was introduced, the Boo Buddy. Also introduced in Super Mario World were the Big Boo and Fishin' Boo, a ghost like form of a Lakitu.
Super Mario 64
Boo's main appearance in Super Mario 64 is in the level Big Boo's Haunt from Super Mario 64. A group of Boos appear in the Castle Gardens, and a Big Boo can be found in the hallway leading out to the gardens. After obtaining twelve Power Stars, Mario is able to enter Big Boo's Haunt. The level portal is found in a small cage inside a bigger Boo. To get to the portal, the Boo must be defeated. When Mario looks at a Boo, they try to hide themselves by becoming partially invisible, instead of hiding their faces, while they're semi transparent, a coin or a Blue Coin can be seen. A punch or a kick is required to defeat them, they are also vulnerable by ground pounds.
Super Mario Bros. Deluxe
Boos may not have appeared in Super Mario Bros., but they make an appearance in the remake Super Mario Bros. Deluxe as the main enemy in "You VS. Boo" mode. The goal is to reach the flag pole before Boo. After so many times, Boo gets replaced by Green Boo. Green Boo is faster, and once Green Boo is defeated so many times, it gets replaced by Red Boo. Red Boo is significantly faster than Boo, and slightly faster than Green Boo. Once Red Boo is defeated, it gets replaced by Black Boo, the fastest Boo in the game.
Super Mario Sunshine
Boos appear in Hotel Delfino on Sirena Beach in Super Mario Sunshine. Boos can be stunned with F.L.U.D.D. and then jumped on for defeat. They disguise themselves as coins that don't spin. King Boo can be found in the basement of the Hotel Delfino. Boos are also able to disguise themselves as Shadow Mario to confuse Mario when he's looking for the real Shadow Mario. Red Boos also make an appearance in which they can be sprayed with F.L.U.D.D and used as platforms.
Super Mario 64 DS
Boos keep the same role as they did in the original. Mario can find a painting of Luigi which takes him to the Big Boo Battle on the balcony. After following the cackle of King Boo, Mario arrives in a room with a mirror in it. To defeat King Boo, Mario must view him through the mirror, since King Boo is invisible. Once King Boo is defeated, Luigi is let free. Yoshi cannot defeat them, for ground pounding has no effect, and if he tries to eat them he will spit them out.
New Super Mario Bros.
Boos appear mainly in Ghost Houses throughout the game, and also in World 8-1. They have the same tactics as those of Super Mario Bros. 3. Two new forms of Boos are introduced in this game, Balloon Boos that inhale air whenever Mario looks at them, and Broozers, the boxing ghost.
Super Mario Galaxy
Boos are found in four galaxies in Super Mario Galaxy, those being the Ghostly Galaxy, the Bigmouth Galaxy, the Deep Dark Galaxy, and the Sand Spiral Galaxy. They act as they did in previous games. They can only be defeated if they are under a stream of light. If a Boo catches sight of Mario while he's Boo Mario, they start moving towards him, falling in the "love" as the heart up them when seeing Mario would mean. Boos also appear underwater in the Bigmouth Galaxy, while underwater, they swim back and forth much like Cheep Cheeps. A new form of Boos are introduced in this game, called the Bomb Boo.
New Super Mario Bros. Wii
Boos reappear in New Super Mario Bros. Wii and appear in Ghost Houses, like in previous games. They can only be defeated by a Star. Big Boos and Broozers appear, as well as Circling Boo Buddies. Their appearance is more of a glowing blue instead of the normal white.
Super Mario Galaxy 2
Super Mario 3D Land
Boos also reappear in Super Mario 3D Land, where they are commonly found in Ghost Houses. There is also a new variety of Boo known as Tail Boos which have Tanooki tails, and will swing them if you look at them before they cover their faces.
Paper Mario: Sticker StarBoos appear only in one level in Paper Mario: Sticker Star: The Engimansion. They invaded the Steward's mansion whom he enlists for Mario's help to capture them all. There are 100 Boos which Mario must defeat and capture all in the Book of Sealing. Some Boos hide cleverly requiring Mario to do a little a more exploring. There are also unique types of Boos including the Disco Boos, Golden Boo, Crumbled Boo, and Boo Stack. After Mario captures all of the Boos, he returns the Book of Sealing into the basement. However, it is soon revealed that Kamek was the one who released the Boos who he thought the Book had rare Stickers in it. Kamek uses his magic to combine all of the 100 Boos into one Big Boo. Mario defeats the Big Boo and the Steward's mansion is soon ghost-free.
New Super Mario Bros 2
Mario Party series
Boos appeared throughout Mario Party series. In Mario Party 1, 2, and 3, Boos appeared in several mini-games. In Mario Party 4, Boo was a host, and owned a board named Boo's Haunted Bash. Boo appeared as a playable character in Mario Party 5, and Pink Boos took over the shops and the mini-games. In Mario Party 7, Dry Bones was Boo's partner, and they both shared the same special item. Boo appeared in Mario Party 8 again as a playable character and he even ran a shop in King Boo's Haunted Hideaway. Boo was removed from the playable character roster in Mario Party DS, although he did appear in a couple of mini-games. Boo appears in Mario Party 9 as an enemy that will take mini-stars away from the captain of the magic carpet. In Mario Party 1-4 Boos are used to steal coins or a star from an enemy for a fee.
Mario Kart series
In the Mario Kart series, Boo is used as an offensive and defensive item. It first appeared in Super Mario Kart. It would render the character invisible to all other racers and invulnerable to all hazards (except for falls). However, if you would to get hit by the flipper in Waluigi Pinball, you would still get shocked and spin backwards. Boo would also steal an item from a racer on the track if possible. It returned in Mario Kart 64, Mario Kart: Super Circuit, Mario Kart DS, Mario Kart Arcade GP and Mario Kart Arcade GP 2 with the same function, but in Super Circuit, Boo also slowed the leader down and caused him/her to lose coins. On the Broken Pier Boos flied around and would attack the player like the item would, however you could stack multiple on you. The Boo item did not appear in Mario Kart: Double Dash!! and Mario Kart Wii. Strangely enough, those are the only two games to have King Boo as a playable character.
Tag Team Names
+Mario= Soul Bros.
+Luigi= Scaredy Pants
+Yoshi= Hovering Fiends
+Daisy= High Spirited Duo
+Waluigi= Total Creeps
+Toadette= Boo Duet
+Birdo= Unfair Pair
+Dry Bones= Scary Pair
+Blooper= Creature Feature
+Hammer Bro= Bad Vibes
Super Smash Bros. Series
Although Boos did not appear in the Super Smash Bros. games, they do make a small appearance in Luigi's Mansion when a part of the mansion breaks off. Boo also appeared as a collectible trophy and sticker.