Since you have mentioned that Rhaast still knows Kayn needs the necessities - but what about indulgences? A craving for a certain meat, or a taste of alcohol.
( Aaah, it’s so nice when someone reads your headcanons! ;;)
Kayn and Rhaast have a symbiotic existence as of now since both exist in one and the same body and soul – with Kayn, as the original host, currently dominating ever since wielding the darkin’s scythe. This does mean that both can read the other’s thoughts and emotions at times, particularly if it comes along with a strong sensation. Rhaast is, however, better in reading Kayn than Kayn him – more likely due to him being so much stronger than the human Kayn. I had explained as much in my former headcanon.
Now to your specific question; Rhaast does not know all about Kayn. As long as Kayn does not mention anything, for example about a certain preference, and as long as this preference is not accompanied with a strong emotion, the darkin will not notice or know. The darkin is, however, fully aware that humans have such foolish needs. For example, if Kayn ever ate something which he finds extremely disgusting, the sensation of it will be strong enough to be noticed by the darkin. The same does apply when eating food which he overly enjoys; the emotion of joy will be merely too strong to be left unnoticed.
However, Kayn actually rarely ever shows such preferences or dislikes. After growing up under the most inhuman circumstances before Zed took him with him, Kayn never finds food disgusting. He was used to dry, moldy bread for so long that he is used to this kind of taste. Kayn also have very little things he actually likes as he was busy with honing his own skills for so very long. He has the most pleasure during his training or during an actual fight with an worthy opponent – and that is what Rhaast is already aware of. Other than that, how I portray Kayn, Rhaast does not know of more details without any thread to reveal more.
I would also like to speak about another problem rising from this; fortunately Kayn does not have such experiences so far – but if he ever falls in love with someone whilst bearing the darkin’s soul, the darkin would notice this probably much sooner than he would. Seeing an opening to dominate him, in the very end. It’s simply a too strong emotion to hide from an ancient being who is so much wiser than all humans.Rhaast himself only bears his own wrath - the darkin’s whole existence is based upon it, so Kayn is unable to feel more than this. And it often effects him since these sensations are so very strong at times.
