Gate Keepers 21 (ゲートキーパーズ21, Gēto Kīpāzu Nijūichi?) is a six-part sequel OVA series to the Gate Keepers animated TV series and has a darker and more serious plot than its predecessor. Some of the questions left unanswered in the TV series are answered in this sequel. Linking the two series are the characters Reiji Kageyama, Yukino Houjo, and the former Far Eastern AEGIS Headquarters setting, Tategami Highschool.

Source: Wikipedia