Sano Gaku (21) will be playing Kazuraba Kouta, the protagonist who becomes Kamen Rider Gaimu. A relatively new actor, he's appeared in minor roles in the GTO series as recently as last year.
Kobayashi Yutaka (24) will be playing Kumon Kaito, Gaimu's rival, Kamen Rider Balon. He has appeared in the Prince of Tennis stage musical, as well as appeared briefly in Dream High.
Takasugi Mahiro (17) will appear as Kureshima Matsuzane, the Chinese themed green/purple Kamen Rider Ryuugen. You may have seen him recently in a minor role in other tokusatsu series, GARO.