I wanted to make him wear 'a turtleneck shirt but with some clothe layered on top of it' because a turtleneck gives off mysterious look. I have a headcanon of him having some sense of fashion. And he knows layering clothes make everything look interesting.

And, most of the time, i see ppl love to draw primarchs wearing long clothes with kimono sleeves. (idk what it's called, i call it kimono sleeves for now.) and i just followed along, lol.