In Descartes's Meditations, Part II, what is the one attribute that Descartes says belongs to his true nature?

When discussing Descartes's Meditations, it's important to be aware a bit about his method. With the First Meditation, he proceeds to test all possible knowledge claims to increasingly radical levels of doubt, beginning with errors in sensory perception and culminating with his most radical supposition—the potential existence of a malevolent, all-powerful demonic entity whose primary delight is intentionally misleading humankind, so that everything we might think we know is in fact a lie. In so doing, he is looking to find one fundamental piece of knowledge, one incontrovertible claim that must always be true, from which a working system of knowledge can be rebuilt. His answer to this challenge (represented in the Second Meditation) is the Cogito—that the human self is tied up with cognition. "I think, therefore I am." Even if the world is a dream, even if all of reality a delusion, even if there is in fact an all-powerful trickster who's misplaced truth with deception, there must still be the thinker experiencing that delusion. This is the fundamental touchstone for Descartes—the act of thinking, which is tied up with that of the essential self.
From here, we get to Cartesian Dualism. For Descartes there is the mind and there is the body, and these remain distinct from one another (even if they are linked together, so that physical sensations relayed through the nervous system to the brain would be reflected in the abstract sensations experienced in the mind). Still, for Descartes, it is the mind that takes primary significance as the lens through which we experience the world and the lens through which we experience our own existence. It is through thought (and more specifically our nature as cognitive entities) that we are defined.