A Boo is one of King Boom Boo's minions in Sonic Adventure 2 and in "Sonic's Scream Test" of Sonic X. They are red-eyed ghosts with sharp teeth.
Sonic Adventure 2[]
In Sonic Adventure 2, Boos appear in Pumpkin Hill, Aquatic Mine, Egg Quarters, Pyramid Cave, Death Chamber, and the boss level for King Boom Boo. Their only ability is to choke the player, or chase them down and touch them, losing all their rings. Lots of times, Boos will hide in gravestones, pumpkins, and even just the ground and will randomly pop up and act as if it was attacking the player, but within a short period of time, the Boo will disappear. This was mostly likely intended to scare the player.
Boos also appear either much smaller or much bigger (the bigger sized one called 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. With the bigger ones, if the Boo is defeated, 3 more normal ghosts come out, and these 3 must be defeated.
Sonic X[]
In the Sonic X episode Sonic's Scream Test, the Boos are ghosts who live in an ancient buriel ground. The man who built a castle over it laughed when he heard legends that there were ghosts there and said they were nonsense. This enraged the Boos and killed him on the first night he stayed there, but he created a seal on the buriel ground using an hourglass.
Years later, a man who was working on a scene of a movie of Linsey Thorndyke removed the hourglass, setting the ghosts free to abduct Linsey, Ella, Cream, Cheese, Tails and Sonic. They restored their full power when King Boom Boo tricked Amy into putting the hourglass up with the moon side up and one of them posessed her, but Chris turned up the sun side and weakened them, allowing Sonic to save Amy and defeat King Boom Boo.
The Boos combined into a giant Boo, but were defeated by Amy. They others escaped, but teh Boos still had their power due to Chris messing up and putting the moon side up. They are last seen in a window at the castle laughing.