Kotani has appeared in a various number of TV shows and movies. He has also appeared in commercials for Pocky (January 2005) and Asahi Beer (December 17–25, 2004). On October 18 and 19, 2003, Kotani appeared in the 2-day staging of ANGEL29x99.
Kotani played a transvestite in the movie, TKO Hip Hop.
Kotani won the role of Takashi Kawamura, the soft-spoken, dual-personality 3rd year regular of Seigaku Middle School's tennis club, in The Prince of Tennis musicals, Tenimyu. From 2005 to 2006, he became the fourth actor to play Kawamura as a member of the second-generation Seigaku cast. He made his debut as Kawamura on January 8, 2005 in the Side Yamabuki performance in Osaka. That same year, his role as Kawamura was carried into the live adaptation film of the manga.
On March 29, 2006, in the Dream Live 3rd Live concert, Kotani, along with the majority of the Seigaku cast, graduated from their roles. Since his graduation, the role of Kawamura has been played by Kouji Watanabe, Hiro Ogasawa, and currently by actors Teyu Kon and Ikko Chou.
Kotani was able to carry on his role from the musicals to the live-adaptation film of the manga series of The Prince of Tennis. He, along with most of the principal cast members for the Seigaku Regulars, were able to reprise their roles in the movie. The only major casting change was the casting of Kanata Hongo as Ryoma Echizen, the main protagonist.
The Prince of Tennis was released on May 13, 2006 and opened on the tenth spot in the Japanese Box Office.
Kotani will also be starring in the made-for DVD release tokusatsu series, ZanSaber, as a tokusatsu hero. The film will also star Kotani's past co-stars from the Prince of Tennis musicals, Kawai Ryuunosuke and Washimi Ryou.
ZanSaber was released on November 18, 2006.
Kotani's next project was filming for the made-for DVD release movie, Boys Love, which also costarred his former Tenimyu costar, Saito Takumi. In the movie, Kotani plays Mamiya Taishin, a magazine editor that meets high-school model, Kisaragi Noeru (Saito Takumi), during a magazine interview.
Boys Love was released on November 24, 2006.
In 2007, Kotani again took the lead in the theatrical edition of Boys Love, which shared only a title with the 2006 movie, and featured a new story and an otherwise new cast. Kotani this time was cast in the part of Aoi Kairu, a young teacher at an all-boys boarding school, who discovers that his new student, Amakami Sora (Kanno Atsumi) is the same boy with whom he recently had a sexual encounter. The movie again costarred other former Tenimyu actors, this time Kawakubo Yuuki and, in smaller roles, Baba Tooru and Kiriyama Renn.
In 2007, Kotani appeared in the film, Arakure KNIGHT (荒くれＫＮＩＧＨＴ), the live-action movie adaptation of the 1982 to 1988 manga series, Aratanaru Shobaku by Satosi Yoshida. The film was released in April 2007.
Kotani once again acted alongside former Tenimyu castmates, Yuu Shirota, Masaki Kaji, Kousuke Kujirai, and Kenta Kamakari.
Previously, in 1998, a live-action TV series had been made titled Shin Shonan Bakusozoku Arakure Knight (Bomber Bikers of Shonan).
Kotani keeps a personal blog, which he updates occasionally. He likes to be called by his nickname, "Konii," and has stated before in a hm3 Men's Extra vol. 2 Cinepuri interview that Tom Hanks's performance in the film Forrest Gump inspired him to want to be an actor.
He currently updates two blogs one in ameblo , the other in FC blog.
On June 13th 2016, it was confirmed that he married Risa Niigaki, former member of all-girl group Morning Musume after two years of dating.
Kotani is now a member of +Plus, a J-Pop band that usually does opening and ending songs for anime. Their latest anime was Hitman REBORN. +PLUS has released four albums.
2007 - "Going to the Road" was his first solo single, that also included "Irotodori no Sekai".2003 - 年秋公演「ANGEL29X99」にて客演ANGEL
TENIMYU: THE PRINCE OF TENNIS MUSICAL SERIES (as Takashi Kawamura)The Prince of Tennis Musical: Side Yamabuki feat. St. Rudolph (In Winter of 2004-2005)
The Prince of Tennis Musical: Dream Live 2nd (2005)
The Prince of Tennis Musical: The Imperial Match Hyotei Gakuen (2005)
The Prince of Tennis Musical: The Imperial Match Hyotei Gakuen in Winter (In Winter of 2005-2006)
The Prince of Tennis Musical: Dream Live 3rd (2006)
1999 JUNON SUPER BOY Contest - 5th Place