@Multitules I also thought of Sugita because Kou kinda looks like Sugita's character in Kimetsu no Yaiba, but Maeno Tomoaki's a great choice. As for the other two I keep thinking of Yusa Kouji for Ren (because he looks a bit like Shima from Ao no Exorcist) and maybe Kamiya Hiroshi for Kai.
@chonkury
omg yess! Yusa Kouji is such a perfect fit. Like, i can't remember Ren's hair colour but i can totally see the pink on him. Absolutely flawless.
For Kai i'm not sure. Kamiya Hiroshi is a very versatile seiyuu so i don't doubt he'd make it fit, but i was somehow imagining a different type of tone for him, like flatter. But i can't think of anyone convincing right now....NojiKen? idk
@Multitules Ren is like a grown-up Shima; they both have pink hair, and Shima's first name is Ren too. So yeah, I immediately thought of him (and Yusa Kouji's version of him) when I first saw Ren.
There are so many seiyuu who could voice Kai (eg, Ono Kensho might be a good choice) that it's hard to decide; Kamiya Hiroshi was just the first person I thought of. I guess I'm okay with whomever gets chosen if ever this gets an anime, they might go with some newer or lesser-known seiyuu anyway. ^^;;