Gate Keepers 21 is a Japanese six-part sequel OVA series to the Gate Keepers series, possessing 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 High School.Thirty-one years have passed since the Far Eastern Branch of AEGIS and the Invaders waged war in Japan. The heyday of growth and prosperity have passed, the crime rate has increased, environmental problems have worsened, and the international situation has become increasingly unstable. Coupled with the decline in the morals of the Japanese people comes the emergence of a new type of Invader — one that can absorb ordinary humans into its ranks.Though they have been officially disbanded and discredited, the remnants of AEGIS have formed a clandestine network to hunt down the Invaders, led by the antisocial Ayane Isuzu, who collects the crystal remains of Invader in exchange for money doled out by the mysterious Reiji Kageyama.
Badass Bookworm: Ayane Isuzu is quiet, ranked third in the second year level of her high school, spends most of her time reading and sitting in front of her laptop. She is also the leading cause of Invader death by blunt cell phone trauma. She also happens to wear...
Equivalent Exchange: Gates run on this. Kageyama points out the fact that most Gatekeepers have suffered some kind of trauma earlier in life. Furthermore, Kageyama's own Dark Gate has, over the years, eaten most of his insides, as well as a literal arm and leg, forcing him to take extensive medication and use Artificial Limbs, the reason why Yukino never ages is because her Gate has consumed her 'time', and he also speculates that Yurei Shoujo's has already consumed her entire body.
Ayane is afraid of being consumed by her gate, and so uses false gates until told that they simply don't have the power needed.
Powers as Programs: Advancements in technology have allowed members of AEGIS to emulate Gate power, encoding them into cell phones. This allows even non-Gatekeepers to have a fighting chance against the Invaders.