X21

 (read as X Nijuuichi) was an idol group originally made up of 21 finalists of the 13th Japan Bishoujo Contest. They disbanded on November 30, 2018.

Final Line-Up

 * Yamasaki Saaya (山﨑紗彩)
 * Matsuda Rina (松田莉奈)
 * Komoriya Sakura (籠谷さくら)
 * Tanaka Shuri (田中珠里)
 * Nagao Mami (長尾真実)
 * Igashira Manami (井頭愛海)
 * Osaki Ichika (尾碕真花)
 * Shiratori Hasumi (白鳥羽純)
 * Kaminaguchi Honoka (上水口萌乃香)
 * Yamaki Koharu (山木コハル)
 * Suenaga Mai (末永真唯)
 * Ozawa Nanaka (小澤奈々花)
 * Inuzuka Shiori (犬塚しおり) (Joined April 2016)
 * Kadogaki Hikaru (門垣ひかる) (Joined April 2016)
 * Kawaguchi Yurina (川口ゆりな) (Joined April 2016)
 * Nakazato Megumu (中里萌) (Joined April 2016)
 * Fujie Moe (藤江萌) (Joined October 2017)

Former Members

 * Nishikawa Misaki (西川美咲) (Graduated April 2014)
 * Wakayama Ayano (若山あやの) (Graduated March 2016)
 * Sera Misaki (瀬羅美咲) (Graduated March 2016)
 * Sagawa Miyu (佐川実優) (Graduated March 2016)
 * Hosoi Yurika (細井友里加) (Graduated March 2016)
 * Takahashi Runa (高橋るな) (Graduated March 2016)
 * Izumikawa Miho (泉川実穂) (Graduated April 2017)
 * Onishi Aguri (大西亜玖璃) (Graduated April 2017)
 * Yoshimoto Miyu (吉本実憂) (Graduated September 2017)
 * Iijima Mika (飯島未賀) (Joined April 2016, graduated March 2018)
 * Takamura Yuuka (髙村優香) (Joined April 2016, graduated March 2018)

Albums

 * [2015.04.29] Shoujo X (少女X)
 * [2017.03.15] Beautiful X

Singles

 * [2014.03.19] Ashita e no Sotsugyou (明日への卒業)
 * [2014.06.25] Koi suru Natsu! (恋する夏!)
 * [2014.09.24] Happy Appli (ハッピーアプリ)
 * [2014.12.03] X Gift (Xギフト)
 * [2015.09.23] YOU-kI no Parade (YOU-kIのパレード)
 * [2015.12.02] Magical☆Kiss (マジカル☆キス)
 * [2016.03.30] Yakusoku no Oka (約束の丘)
 * [2016.07.27] Natsu dayo!! (夏だよ!!)
 * [2016.12.21] Kagami no Naka no Parallel Girl (鏡の中のパラレルガール)
 * [2017.08.16] Genjitsu kara Nigeru kara Genjitsu ga Tsurainda (現実から逃げるから現実がツラいんだ)
 * [2018.03.14] Friday Night Party
 * [2018.09.19] Destiny (デスティニー)