Four

4 is (fittingly) the host of Season 4, Battle for BFDI. 4 has very absurd mannerisms, like being able to deform characters, revive them, and screech in order to stun characters. 4's first appearance in BFDI is Getting Teardrop to Talk, where they lead a game to win "a BFDI" (in which its specifics are currently unknown, which was likely the joke). 4 appeared alongside X, another abstract being representing a real life number or variable.

4's first appearance was in a 2008 animation X Finds Out His Value along with X, nine years before first appearing in BFDI.