From A Wiki of Ice and Fire
The Great Other is the god of darkness, cold, and death in the faith of R'hllor. His true name is never spoken. He is considered the enemy of R'hllor, the Lord of Light. Followers of R'hllor believe that there are only two gods, R'hllor and the Great Other, who wage an eternal war over the fate of the world. All forces of darkness, cold, and death are believed to be only servants to the Great Other. Melisandre refers to the Others as the "cold children" of the Great Other. 
When gazing into the fires, Melisandre sees a wooden face, corpse white, a thousand red eyes, and a boy with a wolf's head beside him. She believes these to be the Great Other's champions, as Stannis is hers.
Further evidence in the text ties the Last Greenseer, Bloodraven, the Old Gods, and Bran Stark to the theme of darkness that draws The Great Other into opposition with R'hllor. The vision of Melisandre pointing towards Bloodraven and Bran as champions of The Great Other is corroborated in this exchange:
There he sat, listening to the hoarse whispers of his teacher. 'Never fear the darkness, Bran.' The lord's words were accompanied by a faint rustling of wood and leaf, a slight twisting of his head. 'The strongest trees are rooted in the dark places of the earth. Darkness will be your cloak, your shield, your mother's milk. Darkness will make you strong.' (A Dance with Dragons, p450)
In an apparent paradox, on one hand the attractive Melisandre and her "Lord of Light" R'hllor clearly display "evil" practices such as human sacrifice in their magic. On the other hand, the twisted Bloodraven and broken Bran (a universally sympathetic character) use the darkness as their ally to merge with nature and apparently aid the forces of "good." However, the true motivations of Bloodraven and the Children of the Forest are not yet apparent. Such devices of anti-achetype are hallmarks of the series.