After UFC 2's launch week, the game has been updated with three new fighters. Two of these were the previously announced Sage Northcutt and Jessica Aguilar, but the third player is not simply added to the roster.
To unlock this third and secret fighter, you need to input the famous Konami code from the game's main menu (Up, Up, Down, Down, Left, Right, Left, Right, O, X, Options on PS4, or Up, Up, Down, Down, Left, Right, Left, Right, B, A, Menu on Xbox One). If done successfully, you will then hear the secret fighter belt out his trademark "WOOOOOOW!! I LOVE IT!!" line. If you still haven't figured out who the secret fighter is, it is the current UFC commentator Joe Rogan.
This means that you get to hear Rogan talk about himself kicking ass or getting his ass kicked. He carries an 86 overall rating and is a Welterweight, and he's worth unlocking whether you'd like to fight with him or just give him a good beating.
If you would like to see Joe Rogan in action, check out the video by YouTuber Z3E3E3 below.