Four

Four is (fittingly) the primary host of Season 4, Battle for BFDI. Four has very absurd mannerisms, like being able to deform characters, revive them, and screech in order to stun characters. Four's first appearance in BFDI is Getting Teardrop to Talk, where they lead a game to win "a BFDI" (in which it means the winners will get another battle for Dream Island). Four appeared alongside X, another abstract being representing a real-life number or variable.

Coverage
Four's first appearance was in a 2008 animation X Finds Out His Value along with X.

Nine years later, Four and X appear in BFB as the hosts. Unlike his original appearance, he acts extremely odd and screeches at others who try to talk to him, making them faint, become dizzy, or even die (in the case of Pillow). The only character who can actually communicate with him is Dora, who also only talks in a strange manner.

Trivia

 * He appears to have four fingers on each hand, however, when he is grabbing Black Hole before the intro, he has five.
 * He appeared in the HTwins.net game Tidepool, as a level 23 creature.
 * Cary described Four as a "weird alien creature who doesn't know how to communicate" in his reaction video. If he actually is an alien, this could explain his strange powers, general behavioral quirks, and different eye and limb assets.
 * He explains challenges in a way where the problem is shown (performed by X, sometimes himself) and he instructs the objects to the task as a challenge.