Park Hae-young (Lee Je-hoo) vs. Saegusa Kento (Sakaguchi Kentaro)
Cha Soo-hyun (Kim Hye-soo) vs. Sakurai Misaki (Kichise Michiko)
Lee Jae-han (Cho Jin-woong) vs. Ooyama Takashi (Kitamura Kazuki)
Kim Bum-joo (Jang Hyun-sung) vs. Nakamoto Shinnosuke (Watabe Atsuro)
Ahn Chi-soo (Jung Hae-kyun) i.e. the crooked cop working for Kim Bum-joo vs. Iwata Kazuo (Komoto Masahiro)
Kim Gye-chul (Kim Won-hae) i.e. the veteran cop who is a specialist in searching for people vs. Yamada Tsutomu (Kimura Yuichi)
Jung Hun-gi (Lee Yoo-joon) i.e. the cop in Hae-young and Soo-hyun's team who is an expert in forensics vs. Kojima Shinya (Ikeda Tetsuhiro)
Oh Yoon-seo (Jung Han-bi) i.e. an assistant professor in pathology who is Soo-hyun's buddy and informs her whenever an unidentified body is found vs. Anzai Rika (Aono Kaede)
Park Sun-woo (Kang Chan-hee) i.e. Hae-young's late elder brother vs. Kato Ryota (Kamio Fuju)
Personally, as "Signal" is one of the few Korean dramas I like, there are big shoes to fill for the Japanese version in order for the latter to impress me in the same way. And I know that I'll probably keep comparing between the two which is probably better if you haven't watched the Korean version and can enjoy the Japanese version for what it's worth. Especially since the Korean version's ending was rather open-ended, I wonder how the Japanese version will be wrapped up.
Casting-wise, I don't have much issues with it but Kichise has never had a role so far which truly caught my eye other than the straying housewife in "Hirugao". As such, I have reservations about whether she can cope with this difficult role who is the bridge between Saegusa and Ooyama and has serious emotional acting to tackle. And because Kitamura and Kichise had such a sizzling chemistry in "Hirugao", it might be a bit difficult to stop myself from remembering those "hot" scenes when I see them together in this drama. Watabe as the villain should be nothing challenging for him because he's great at such roles as always.