I got interested in Ghost of Tsushima and Sekiro and I got too into Japanese clothing and the armors. It feels sci-fi with the plugsuit but I wanted to put a tight fitting clothing on Byleth for modesty's sake in case she ends up ripping the kimono off.
I didn't really have anything particular going on with this design, I just thought it had to feel weird just as Byleth's original clothing, and I had to fit in a kimono at least.
The pattern of the kimono is mist, spider lilies and butterflies (though it wasn't obvious). I think if you look up the symbolism of the spider lilies and butterflies you'll find something interesting about them.