Edit: So how about a bit of trivia now that I see that you've finished your drink.
Takahashi Ryuki who plays Kengo on Fourze was nearly Gentarou. He made it to the final audition for the role along with Fukushi Sota who got the role. Takahashi is also a D Boy so he was in the Prince of Tennis Musicals. Plus he was on High Kick Girl, as well as movies with other tokusatsu stars like the movie "Messiah" and "Mahou Shoujo wo Wasurarenai".