Shaco

Shaco The Demon Jester was first seen in the second official trailer as eye-candy for game footage. Then announced in Incgamers.com along with Heimerdinger

Background
They say that death isn't funny. It isn't, unless you're Shaco. He is Valoran's first fully-functioning homicidal comic; he jests until someone dies, and then he laughs. The figure that has come to be known as the Demon Jester is an enigma. No one still alive knows from whence he came. There are rumors, but one must not speak of them too loudly lest Shaco come to jest. It seems unimaginable that such a threatening figure would be allowed to remain at large. There are those that believe he is an assassin for hire, left to roam until he is needed. Others believe that Shaco is simply too clever, evading authorities at every turn. Still others believe that he is the demonic manifestation of humanity's dark urges, and therefore cannot be stopped.

To jest with Shaco is to box with shadows. His mastery of the shadows is unparalleled, limited only by his magical reservoir. This allows him to deal devastating amounts of damage as he stabs his enemies in the back. Shaco can also confound his enemies by creating an illusion of himself that will do combat, only to violently explode when its time is up. The wicked daggers he carries are coated with the same ajunta poison gathered by the yordle Teemo, bringing pain, suffering, and death. To complete his ensemble, Shaco also plants psychotic animated Jack-in-the-Boxes on the field of battle, which pop out and shoot at nearby foes.

Whatever you do, don't tell him you missed the punch line.

Trivia
He was only known by Jester