Boo

Boos (ブー) are white ghosts, usually seen inside of pyramids or any other haunted zones. Boos are also known to be servants of King Boom Boo and have made their first appearance in Sonic Adventure 2. In that game, Boos come in two variations. The normal one, which float around with black pupils and their mouths seen as folded fabric thread. The second type of Boos have large red/orange eyes with sharp black teeth.

Sonic Adventure 2
Debuting in Sonic Adventure 2, Boos are seen appearing mostly at Knuckles's action stages such as Pumpkin Hill, Aquatic Mine, Death Chamber, and even during the boss battle against King Boom Boo. However Boos also appear at Sonic's action stage Pyramid Cave and even teasing Rouge the Bat at Egg Quarters. They are seen floating around the ground, but they also hiding in gravestones or pumpkins. Normal type of Boos will randomly pop up and act as if it was attacking the player, but within a short period of time, the Boo will disappear. The player will not lose Rings, but got stunned. When normal types of Boos disappear, they will not earn points.

There are also attack types of Boos, which are able to hurt the player like regular enemies. These type of ghosts can either choke the player, or start chase them down the player while also sometimes seen jumping on to him. Getting touched them, the player will lose all rings. After hitting this type of Boos will give 100 points to the player. Throwing objects to these enemies will not affect on them. Boos also appear either much smaller or much bigger (the bigger sized one is named a Boom Boo), with cuter eyes and a patched up mouth. But this should not deceive the player, because the small ones will randomly surprise the player with a quick attack and then disappear. None of Boo types contain animal friend or any Chaos Drives to collect.

Sonic Lost World
Both Boo and Boom Boo variations will make appearance in upcoming Sonic Lost World, where they can be seen featured at Sky Road. What is only known is that, the player has to avoid them by using Magenta Wisp's Color Power, while all of Boo and Boom Boo ghosts are seen moving on large rows to the left direction.

Physical Appearance
Boos are known to be ghosts, as they have white body round shaped body. Normal type of Boos have black, soulless eyes on face while also having it's mouth closed, looking like being knitted up. These Boos also have small thin hands and are known to have two small sharp ends below the round-body, which can supported legs hovering around.

Attacking type of Boos are seen to be larger and having red and orange colored maniac eyes with black pupils and surrounds on it's eyes. It's hands are more sharp with three ends on each of them and the two ends below the body are transformed to the one. It's mouth also is seen to be opened, that shows to have sharp black teeth. These type of Boos are seen to be much energetic than normal types.

Attacks and Abilities
Normal types of Boos are seen hiding in gravestones, pumpkins, and even just the ground. These types of Boos will suddenly pop up to it's victim's face and attack to him. It however doesn't in the end happen, as it can make it's victim stun, it will after for while disappear in the air. These types of Boos don't hurt, but are able to scare the player in the most sudden moments possible.

Attacking types of Boos have two different types of attacks it can perform. One of them includes holding the playable character in place and make another Boo or other regular GUN or Eggman's robots attack the player. They can be seeing floating around normally, until it discovers the player and starts chasing the player until it runs over him.

Sonic X
Both normal and attacking types of Boos along with King Boom Boo have made appearance in episode "Sonic's Scream Test" of Sonic X anime, where Boos behave differently from their game counterparts. They also live inside the old, abandoned castle in the middle of the lake instead of Eggman's large pyramid base. In the episode, the Boos made many laughing noises inside the old castle. Instead of choking others to attack, they possess the host (in this case Amy Rose was the victim).

During the episode, Sonic and Co. has arrived to the castle to see Lindsey Thorndyke's latest movie being under the filming production, while one of cameraman of the filming group takes hourglass statue from it's place, which opens the portal inside the castle and releases Boos out of vortex with unknown energy during the night time. During the first night Boos start to appear around the sound editor of the filming group and Lindsey Thorndyke, both of them disappearing, as Boos have take them to the vortex.

As finding inside the castle, Amy, Cream, Cheese and Ella found the rock door to the vortex. However Boos start to take them to the vortex as well, while Tails and Chris shows up and manage to save Amy. However Cream, Cheese and Ella got sucked in, Sonic arrives with Hourglass on his hand and tries to attack Boos, who appeared behind the rock door. He misses, but manages to break the rock door, revealing vortex in front view. However Tails and Sonic got caught by Boos, but Sonic manages to throw hourglass statue to Chris, and tells to him to return to it's altar. The rock door starts to fixed itself, thus logging the path to vortex.

Next day, Chris and Amy find out the another path to the vortex under the hourglass statue pillar. Boos start attacking them after arriving long hallway, but got scared away by hourglass statue, which Amy hold on her hands. While Amy and Chris noticing this, they start follow Boos to the wood door, that leads to below the vortex. In the room, they see Sonic, who tries to warn them, but his voice gets manipulated by King Boom Boo, which Amy and Chris don't notice. King Boom Boo guides them to put hourglass statue to the small hole with Moon symbol on the statue facing upwards. For astonishment of Amy and Chris, King Boom Boo and Boos got released out the vortex completely. While Amy gets caught by King Boom Boo, one of Boos starts take control of her mind. During this, Amy's eyes and mouth change to similar ones of Boos. Amy thus starts attacking to Chris, while other Boos keep laughing to them. Chris notices that Boos can still scare the Hourglass Statue despite being set on the hole. He changes the pattern by switching the Sun symbol on the statue to face upwards. This causes Sonic and others get released from the vortex, while Sonic karate-chopping Boo out Amy's body and defeating all Boos easily. Amy joins the fight as well, while also five Boos start to fuse to the larger Boom Boo, which still gets easily defeated. Finally King Boom Boo and all Boos to get locked up the castle after everyone has escaped under the castle and Chris inserts the hourglass statue back to it's place.

Trivia

 * Before Sonic Adventure 2, there have been similar ghosts enemies called Hyudoros, which act similarly like Boos and are featured inside the pyramid at the second act of Sandopolis Zone in Sonic & Knuckles. Despite these facts, both Boos/Boom Boos and Hyudoros can be consider as separated ghost species.
 * Boo shares its name with enemies with same name from the Mario series.
 * Boos' name in eyecatch card of Sonic X is spelled mistakenly as Boom.