Romi, Riki's replacement, appears to be Hatsuno Hana aged up and freakin' awesome.
Kazuya Tatekabe is Gian, from Doraemon - if you grew up in Japan with a TV in the 90s, you know what he sounds like. He is also Riki here, and goes as far as to do a Next Episode narration not as Riki but as Gian.
Isao Sasaki sings the opening and ending. Why? Because YES, that's why.