The Nefarious skin is an unlockable skin in two Ratchet & Clank games.
It is a special multiplayer skin in Ratchet & Clank: Up Your Arsenal. It is unlocked via a code received through the Insomniac Games website.
The skin later returs in Ratchet & Clank Future: A Crack in Time, as the last obtainable skin, obtainable after finding all 40 Gold Bolts.
The skin later returns as a DLC in Ratchet & Clank: Full Frontal Assault multiplayer, and is playable after purchasing Villain Pack.