Belial concept art.
Belial, the Lord of Lies, is one of the Lesser Evils (along with Azmodan, Andariel and Duriel) and is the ruler of the Realm of Lies in the Burning Hells. With the help of Azmodan he successfully exiled the three Prime Evils to Sanctuary. Shortly after this, civil war broke out between the two Lesser Evils. Belial ultimately lost this war after just two decades.
It's unknown whether he submitted to Azmodan after his defeat, or indeed whether he will side with his brothers in Diablo III at all, given the animosity he feels towards them.
Belial is yet to be seen in-game, but he is confirmed to make an appearance in Act II. In the concept art he appears to hover, and has a skeletal frame for a body and four eyes.
He's well known for using lies and deception to great effect, and is undoubtedly the most deceitful of the Great Evils, shifting others' perception of reality in order to control their actions and bend them to his will.