Kaa is the name of the snake character in Rudyard Kipling's 1894 anthology collection The Jungle Book. He is a thirty-foot-long python most distinguished by his mystical powers of hypnosis (to which only human beings like Mowgli are immune). He is a century old, still very powerful, and well respected, even feared, by the other animals in the jungle due to his powers. His hypnosis is indeed fearsome and a force to be reckoned with, even luring in allies like Baloo and Bagheera, much against Kaa's intentions.
While most film versions of The Jungle Book, most famously the popular 1967 Disney adaptation, make Kaa an antagonist to Mowgli and his friends (more than likely due to snakes being associated with evil in much of Western culture), in the book he is more of an ally, even a friend. He aids Mowgli and the others in battles in both the first book and in the sequel, The Second Jungle Book. When Mowgli leaves his old life in the sequel, Kaa even encourages Mowgli to be brave, comparing the harshness of moving on from the past to a snake having to shed its old skin. This gives the old snake an almost fatherly air.