''Gate Keepers 21'' is a Japanese six-part sequel OVA series to the ''GateKeepers'' series, possessing a [[DarkerAndEdgier 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.
----
!!''Gate Keepers 21'' contains examples of:
* ActionGirl: Satoka, with an ''arsenal'' of swords.
* AllThereInTheManual: Bad news: You have 31 years of catching up to do between ''GateKeepers'' and this. Good news: There are some novels and a video game bridging the gap. More bad news: [[NoExportForYou None of them are in English.]]
* TheAtoner: Kageyama
* BadassBookworm: 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...
** A BadassLongcoat.
* BerserkButton: Ayane ''really'' likes birds. God help you if you do anything bad to them. Just ask [[spoiler:Count Akuma]].
* BetterThanItSoundsAnime
* BlastOut: Yukino and Ghost Girl move from a MexicanStandOff to this in the final episode.
* BlowYouAway: Ayane's Gate power
* {{BFS}}: Satoka's Gate Power calls forth these.
* BountyHunter: AEGIS employs 'Invader Hunters', who may or may not be Gatekeepers (those who aren't use Imitation Gates) and pays them according to the number of invader crystals they collect.
* BuddyCopShow: Ayane and Miu ({{Student And Master Team}} variation).
* CerebusSyndrome: There are many fans of the original series that don't much care for the DarkerAndEdgier feel of the sequal series.
* ContinuityNod: When Ayane and Miyu launch the [[CoolCar UP-15]] from the [[SelfDestructMechanism self-destructing]] AEGIS base, Yukino does the "Ooh, intense!" thing that accompanied AEGIS vehicle launches in the original series. No {{Panty Shot}}s, though.
* CoolShades: Kageyama
* DarkerAndEdgier: In ''GateKeepers'', you had hope the ThePowerOfLove would prevail. Here, not so much.
* DarkIsNotEvil: Kageyama, though he also happens to have [[LightEmUp elemental light]] as a power, so...
* DemonicInvaders: The Invaders, natch.
* DemonicPossession: Any non-Gatekeeper humans who succumb to ''any'' dark emotion open themselves to possession by the Invaders.
* DisappearedDad: Shun
* TheDitz: Miyu
* {{Dude Shes Like In A Coma}}: Ayane Isuzu is kissed while comatose by BigBad StalkerWithACrush Yuurei Shoujo, while the former's boss and partner watch helplessly.
* TheEndOfTheWorldAsWeKnowIt: [[BigBad Ghost Girl]] causes this in the GrandFinale.
* EquivalentExchange: Gates run on this. Kageyama points out the fact that most Gatekeepers have suffered some kind of trauma earlier in life. Furthermore, [[spoiler: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 ArtificialLimbs, 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.]]
** [[spoiler: 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]].
* EverybodysDeadDave: Almost the entire cast from ''GateKeepers'' die ([[PutOnABus or are simply written out]]) in the backstory.
* EverythingsBetterWithSparkles: Miu's Gate of Leap gives off quite a lot of them.
* GratuitousEnglish: "This is your power. It's a miracle!"
** GAAAYY-TOOOOHH OHHH-PUUUNN!!!
* HammerSpace: Where Satoka keeps all her PureEnergy [[CoolSword Cool Swords]].
* ImprobablyFemaleCast: The only male characters are a SupportingLeader, the two Dragons, and an InspectorJavert who doesn't seem to serve any purpose other than {{padding}}.
* InASingleBound: Miu's Gate Power is called Flight but is really more of this trope.
* InconsistentDub: The English dub of the TV series refers to Akuma Hakushaku as "Baron Akuma." In this one, he is called "Count Akuma" instead.
* MagicalGirl: played pretty straight with the Gatekeepers, but also parodied by a "film-in-film" Miu and her friends watch.
* {{Meganekko}}: Ayane
* MoodWhiplash: In contrast to the previous anime, don't expect much in the way of bright and happy. They ''really'' made sure that the days of the "original" Gatekeepers were long over.
* NumberOneDime: Ayane's bell. Depending on how her surname is spelled in Kanji, could veer into MeaningfulName territory, as one spelling of "Isuzu" means 'Dependable Bell'.
* OddlyNamedSequel
* OlderAndWiser: Several characters from the original ''GateKeepers'', now grown up, act as commanders, mentors, and parents to the new cast.
* OneWomanWail: The opening theme.
* OurHeroIsDead: At the end of episode 4 of ''GK21,'' [[spoiler: Ayane [[WhyDontYaJustShootHim is shot from behind]] on the way home from killing Count Akuma.]]
* ParentalAbandonment
* PlantAliens: Count Akuma has now become this
* PowersAsPrograms: 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.
* PsychoLesbian: Yurei Shoujo, the BigBad, ''totally'' has it in for Ayane. See DudeShesLikeInAComa above.
* ReallySevenHundredYearsOld: Yukino and Yurei Shoujo.
* ResetButton: [[spoiler:The GrandFinale]]
* SailorFuku: Miu brings Yukino a ''seeraafuku'' for a school uniform. Why not get her ''an actual uniform'' of Miu's school? Well, [[RuleOfCute because]].
* ScaryShinyGlasses: Ayane Isuzu.
* ShoutOut: The crane game has characters from ParappaTheRapper, Manga/AzumangaDaioh, and KeroroGunsou, among others.
* SpiderSense: Kageyama uses his Gate of Foresight to make lots of money in the financial world, which he uses to fund AEGIS and pay Invader Hunters.
* TheStoic: Ayane
* StrongFamilyResemblance: Ayane bears a resemblance to her aunt Saemi, Shun's younger sister, except [[{{Meganekko}} she wears glasses]]. Saemi furthermore resembles her mother after growing up.
* ThoseTwoGuys: Miyu's friends, Chinami and Nao-chan.
* UnknownRival: Tachikawa Satoka spends most of her free time talking to[=/=]critiquing[=/=]lecturing BadassBookworm Isuzu Ayane only to get ignored, or ditched when her back is turned, much to her chagrin.
* TheVirus: The Invaders.
* VaporWear: Yuurei Shoujo.
----