Nitro Zaust is an ancient spirit reincarnated as a Mask. He supports various Crash Bandicoot characters in races who are neither on Crash / Aku Aku's side, nor on Cortex / Brio / Tropy / Uka Uka's side.
Nitro Zaust first appeared in Crash Team Kart Racing when Papu Papu agreed to follow Nitros Oxide in his quest to seek revenge against Crash, Cortex, and their companions. In return Nitros Oxide would forever banish them from their home planet. Unfortunately for Oxide, Papu, and Nitro Zaust, the bandicoots and Uka Uka's henchmen foiled them, allowing them to continue living on Earth.
Despite being a generally evil character, he will be the patron mask to support Fake Crash Bandicoot, Penta Penguin, the Trophy Girls, and Spyro the Dragon, if they are downloaded from the Internet to Computer [PC], transferred from memory card [PS], data disc [XB360] to console. Even though they are good / neutral characters.
This creates the impression that Nitro Zaust is actually supposed to be a neutral character, even though he is generally portrayed as an evil character.