
Quick answer — Tsuyoshi Aoki voices Kumagai in Rainbow, Inage in Toradora!, Seiji Takasu in Toradora! and 5 more characters.
Tsuyoshi Aoki voices Kumagai (Rainbow), Inage (Toradora!), Seiji Takasu (Toradora!), Si Li (Kingdom), You Long (Kingdom), Daikichi Asahina (Witchy Precure!), Douglas (If It's for My Daughter, I'd Even Defeat a Demon Lord), Fei Ling (By the Grace of the Gods).
Kumagai is voiced in Japanese by Tsuyoshi Aoki.