Kullervo is the antagonist of the series, and Connie Lionheart's companion creature, though the connection is against her will. He is a shapeshifter, therefore he can communicate with other creatures, including humans. Despite his ability to do so, he refuses to shapeshift into a human, expressing his strong dislike for humanity.
Kullervo's natural form is basically a cloud of navy and black mist, and when he shapeshifts into other creatures he is navy-black with mismatched eyes--one acid yellow, the other golden.
He is manupulative, being persuasive enough to turn creatures against humans and taking the path of destruction. He seems to have been looking for a companion for quite a while, and seems to be immortal, seeing he has lived since around when the Society was founded.
He has tried to form a bond with previous universals during World War II, but they were shielded from him, and he was defeated and sent away. He did not stir until Connie came along.