Skullface

Skullface is a new character who debuted originally in Metal Gear Solid V: Ground Zeroes. Although his history is currently unknown his face is covered in scars and he works for (and presumably leads) a militia organization known as XOF. The ties between XOF and FOX are currently unkown.

Theories
Although Konami and Kojima have not currently confirmed Skullface's true identity, the most popular theory is that he is the father of Big Boss. The theory (likely true) goes that Skullface was a warrior himself and became scarred due to a particularly terrible battle. Disappointed that Snake (now Big Boss) is following in his footsteps, Skullface (theoretically) uses war as a means to an end - that end being the end of all war.

Another theory is Skullface is Volgin from Metal Gear Solid 3. The scars on his face hint could offer proof, being that Snake killed Volgin with fire and electricity, and would explain the reason why Skullface would want revenge against Big Boss, as well as why he would title his unit XOF.

Another theory is that Skullface is the Fury. That's a good theory, too.

It has also been hinted that Skullface could be Decoy Octopus, although many have dismissed this as classic Kojima misdirection.

Trivia
In an interview with Famitsu in November, 2012, Hideo Kojima revealed that the original name for Skullface was "Scarface."