Boos are a mischievous, round ghost-like species found in the Luigi's Mansion series. They are known display shy characteristics, covering their faces when an enemy looks directly at them, although this is not the case for all boos, as some of them are very exuberant and confident in their actions against enemies.
Boos are the main antagonists of Luigi's Mansion, acting under the orders of King Boo. There are 50 Boos scattered all over the Mansion, 40 of them each with unique names and dialogue,whilst the remaining 10 make up part of Boolossus.
In order for Luigi to capture a Boo, he has to uncover it's hiding place and drain its HP with the Poltergust 3000 to suck it up.
Luigi's Mansion: Dark Moon
Boos are once again the main antagonists of Luigi's Mansion: Dark Moon, serving King Boo and aiding him throughout the Mansions. They are seen carrying the Mario portrait for King Boo, and assisting King Boo in trapping toads in portraits. There are 32 Boos total, 22 of them being uniquely named Boos scattered throughout the Mansions in hiding. The other 10 are part of Big Boo. In Luigi's Mansion: Dark Moon, King Boo gives the boos the ability to wield spirit balls, which can turn objects invisible.
For Luigi to capture a Boo, he has to shine the Dark Light on a boo, which will cause it to doze off. Luigi can then attach his Poltergust 5000 to the Boos tongue, pull back and let go of the Boo which will cause it to launch and ricochet across the room, before hitting the ground, dazed and confused. Luigi can then suck it up.
In Luigi's Mansion: Dark Moon, boos attacks consist of charging at Luigi whilst invisible, and bouncing off of Luigi if he waits too long to launch the boo when attached to its tongue with the Poltergust.
Luigi's Mansion 3
Boos return in Luigi's Mansion 3, again as the main antagonists. The Boos serve under the orders of King Boo, and hide in the various floors of the hotel. There are 16 Boos you can capture, 1 on each of the 16 main floors.
Boos attacks in Luigi's Mansion 3 consist of rapidly twirling at Luigi, and throwing bombs.
For Luigi to capture a Boo, he has to shine the Dark Light on a Boo, causing it to doze off. Luigi can then attach the Poltergust G-00 to its tongue, and rapidly slam it against the ground to drain its HP before sucking it up.