The rest of the cast has been announced (with some help from Google Translate). A few of these translations are rather amusing.
http://hpot.jp/stage/billyjapan/cast
Michael
Kuruga Ru Castle Nozomi Tsukuda Yuden Yamaguchi Ren
Debbie
Sasaki Kotoka Natsukawa Asahi Deep Sea Honey
Ballet Girls
Endo Mio Masako Okubo Ozono Azusa Kuboi Maiko Sasaki Yoshinori Takahata Mino Konnichiro Tanimi Tsuki Nagisa Nagisato Aino Horikoshi Yuri Ai
Tall Boy
Minoru Konishi Nori Sasagawa Shingo Yamashiro
Small boy
Okano Ringo Kikui Masato Sakurai
Dad
Yoshida Shotaro Toru Masuoka
Professor Wilkinson
Yuzuki Rei Shimada Shingo
Old lady
Ayako Kuno Negishi Kinoku
Tony
Masaaki Fujioka Masataka Nakagauchi
George
Masahiro Kobayashi
Older Billy
Ru Kuriyama (K Ballet Company) Yuusuke Onuki
Moriyama Daisuke
Home Tsukuko child
Otsuka ta ka shi
Kaga real valley
North Village Yi
Sasaki wood honest
Takahashi Zhuo Shi
Chen Bazhi autumn
Hashimoto good Hong
Birds birds Xiang too
The original Shinichiro
Maruyama Thai right
Yokosawa Kanji
Kimura crystal child
Island Amisha
Takeuchi crystal beauty
Miki linen child
Akiyama Aya
Inoue cauliflower
Export child