X

X is a character first appearing in Battle for BFDI. X's first appearance in BFDI is Getting Teardrop to Talk where X and 4 lead a game to win "a BFDI" (in which its specifics are currently unknown, which was likely the joke). X appeared alongside 4, another abstract being representing a real life number or variable.

X played a major role in the challenge in Getting Teardrop to Talk. The contestants had to return a basket back to X in order to avoid elimination.

X's first appearance was in a 2008 animation X Finds Out His Value along with 4, over nine years before first appearing in BFDI. This also shows that X doesn't refer to the English letter, but the algebraic variable.