Zer0 is quite the enigmatic character, with his real name and origins being a complete mystery to pretty much everyone. His nickname comes from the "0" that is displayed on his helmet whenever he sometimes kills an enemy. He is an assassin for hire and will pretty much kill anyone, whether they are an important person or not. He travels to Pandora after hearing about the Vault, so he can test his skills.
Zer0's action skill, Decepti0n, allows him to become cloaked for six seconds, while also deploying a decoy. If you shoot a gun or hit an enemy with a melee attack, then the cloak will disappear. However, your first attack to break the cloak will be much more powerful (up to 200% gun damage and up to 650% melee damage, with a 40% and 230% base, respectively). With the damage increases, this is great for a sneak attack on an enemy, but it can also be used to run away when you're in danger.