@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. ^^;;