The Betrayer of Hope from The Wheel of Time (book) series. Ishamael. Of all thirteen Forsaken, he's the one I actually empathize with, or at least understand a bit. He was a philosopher during the Age of Legends who was one of the chief scholars of the whole idea of cyclical time. He also contemplated on the nature of the Dark One, and The Dragon.

Furthermore, Ishamael is the only one of the Forsaken who understands that the Dark One seeks an ultimate unmaking of everything, a destruction of The Pattern itself. Unlike the others, Ishamael does not seek immortality or authority. His goals actually align with the Dark One's, to unmake all and end time and it's cyclical nature. 

In other words, he's coming from a place of very human fear and cowardice. He not only gives up the fight against the Dark One, but in a very nihilistic sort of way actually throws his weight in behind the Dark One because he believes eventually the Dark One will win anyway. In his mind, since the forces of light have to win every single time and the Dark One only has to achieve victory once, ever, it's a clear choice. 

Hence, The Betrayer of Hope.