One thing I love about the Rhealeth supports, and this this art captures really well, is just how different Byleth and Rhea’s relationship is in the C through A ranks compared to the S rank. In C through A, there’s a wall seperating the two of them. Rhea stands in a position of power above Byleth, both in terms of her status and knowing way more about the world and Byleth herself. Rhea’s interested in Byleth, as both an indivual and potential vessel (or reincarnation or whatever) of Sothis, but Byleth, even though she likes Rhea, is still uncertain of what Rhea really wants from her. In their S support, they’re finally at the same level. Byleth has seen Rhea at her absolute lowest, and finally knows pretty much everything that had been kept from her. And even then, she still loves Rhea and wants to be with her, who now loves Byleth as herself, rather than what she thought she could be. The vulnerability that Rhea shows Byleth and the emotional intimacy that comes with it, when contrasted with the first three supports, is part of what makes the Rhealeth S support the best IMO.