From the Talmud, the demonic angel of the sea. Rahab is or was the angel of insolence and pride, responsible for shaking the waters and producing big waves; he is also responsible for the roaring of the sea.
In Jewish folklore, Rahab (noise, tumult, arrogance) is a mythical sea monster, a dragon of the waters, the "demonic angel of the sea". Rahab represents the primordial abyss, the water-dragon of darkness and chaos, comparable to Leviathan and Tiamat . Rahab later became a particular demon, inhabitant of the sea, especially associated with the Red Sea.
Rahab is an Angel that resides in Rahab no Ibashyo, where he watches over one of the books of Raziel. Hearing Masaki Kuzuha's name has him reveal that he has been waiting for him, and offers him a chance to read the book and learn of his future. He tells him that there are other books as well once he takes it and that he should collect them all.