Gate Keepers

Gate Keepers started out as a Turn Based Strategy (with Dating Sim elements thrown in) video game for the Playstation in 1999. A manga was also commisioned as well and both were released in the same month. It was later adapted into an anime series in 2000 by GONZO who decided to omit one of the main Gatekeepers and replace another with another character with a similar design. The setting takes place on Earth, after the end of World War II. In the middle of all that, alien creatures known simply as Invaders are infiltrating Earth and attempting to take over the world by turning humans into soulless robots. The only means to defeat the Invaders is by tapping into powerful energy locked inside dimensional "gates".

That's where a secret agency known as A.E.G.I.S, short for "Alien Exterminating Global Intercept System," comes in. They seek out people known as Gate Keepers, who have the ability to unlock the gates, which in turn gives them superhuman powers. The protagonist, Shun Ukiya, is one of the Gate Keepers, who becomes aware of his powers after witnessing a fight between the Invaders and a schoolmate -- and childhood friend -- Ruriko Ikusawa. The story goes on to include other Gate Keepers and their exploits as they fight against the Invaders in their bid to defend Earth.

It also spawned a sequel, Gate Keepers 21, set 31 years later. There is also a novel, set in 1985, which helps explain some of the events between the two anime series.

If you were looking for the young adult series of novels about the Gatekeepers, see The Power of Five

This anime contains examples of:

 * Action Girl: Ruriko, Kaoru, Yukino, Feiling.
 * The Ace: Jim Skylark.
 * All Amazons Want Hercules:
 * All Your Base Are Belong to Us:.
 * Alternate History: It's mentioned that the Invaders were first encountered in 1945. The events that transpire in the show itself, as well as the existence of AEGIS, could also count. Though by 2001 (Anime/Gatekeepers21), history has more or less become similar to our own, if not worse...
 * An Ice Person: Yukino
 * Bait and Switch Credits: Subverted. One scene from the OP apparently depicts Megumi firing a ki blast from her hand (which is naturally impossible due to the nature of her powers, and thus, wouldn't happen). It turns out she was using her powers to block that energy blast, not to produce it.
 * Barrier Warrior: Misao, in the games. Her Expy Megumi, in the anime.
 * Beach Episode: Which is actually sort of plot relevant
 * Berserk Button: One surefire way to piss Ruriko off is to call her a 'sniveler' (or just refer to her childhood being a sniveler). Though Shun does use it as an advantage at times.
 * The Big Guy: Bancho
 * Blow You Away: Shun's power, as well as that of his daughter Ayane,.
 * Body Horror: Two different types, depending on the series. In the case of Gate Keepers, the Invaders were sleeper agents, some sleeping so deep that they were horrified by the realization of what they really were. In Gate Keepers 21, the Invaders are The Virus.
 * Boisterous Bruiser: Bancho.
 * Brainina Jar: Count Akuma's brain is in, for lack of a better word, a fishbowl.
 * Brainwashed and Crazy: in the final episode.
 * Calling Your Attacks
 * The Cavalry: Jim and his team in the final episode.
 * Chekhov's Gunman: That woman wearing the helmet and goggles whose skirt always gets blown up is actually.
 * Chinese Girl: Feng Fei Ling (as if that's not obvious enough). Even her invocation was in mandarin.
 * Cool Shades: The AEGIS Commander
 * Crazy Prepared: AEGIS just happens to have a giant piano for the Gate Robot in case Reiko decides to pilot it instead of Shun.
 * Cute Bruiser: Kaoru
 * Delinquents: Shun and Ruriko disguise themselves as delinquents (Shun wearing his uniform in disarray, Ruriko dressing like a sukeban with a long skirt and curly hair) in order to confront Bancho -- whose nickname itself is Japanese for "delinquent."
 * Disappeared Dad: Shun's father, Also, Shun himself in Gate Keepers 21,
 * The Ditz: Reiko, though this might be out of trauma after her parents's divorce.
 * The Dragon: Kaiser Kikai and Count Akuma.
 * Driven By Envy: Ruriko is popular, beautiful, rich and kind. Megumi is socially-inept, plain-looking, middle-low class and embittered.
 * Everyone Calls Him Barkeep: The Commander. His name is revealed in the novel.
 * Evil Counterpart: "Shadow"
 * Evil Makeover: Happens to whenever he switches to his "Shadow" identity. Inverse Gates are pure black, regardless of their original color.
 * Expy: Megumi is a strange case in that she coexists with the person she is expying, Misao Sakimori.
 * Face Heel Turn:
 * Fun With Acronyms: Alien Exterminating Global Intercept System.
 * Genki Girl: Francine although she doesn't reach the heights that many of the other characters do for the trope and can be very serious when she wants to be but her goal in life is to make one thousand friends.
 * Gratuitous English: Any American character who isn't part of Jim's team. Also Francine from the game.
 * Healing Hands: Ruriko
 * The Heartless: The Invaders
 * Heel Face Turn:
 * Hopeless War:
 * How Do I Shot Web: Shun struggles in every episode to learn how to use his ability properly.
 * Humongous Mecha: Five of them, actually. In 1969.
 * Instant Fanclub: Reiji has at least three girls as members of an actual club that he started.
 * Invocation: "Gate! Open!"
 * Jerkass: Most of the undercover Invaders are like this in their human disguise. One episode that centers around one shows that this may partly be due to stress as it was mentioned that the invader only started acting like Jerkass shortly before the episode.
 * Lady of War: Ruriko, who uses arrows, as opposed to Kaoru, who uses her fists.
 * Lamarck Was Right: Shun's daughter Ayane inherits her gate power from him. Furthermore.
 * Leitmotif: In the game each of the main female characters and some of the supporting characters have their own theme.
 * Lotus Eater Machine
 * Magic Music: Reiko's Gate power is focused on her piano playing, which creates beautiful illusions for people... and acts as a Brown Note for invaders.
 * Master of Illusion: Count Akuma, and to a degree Reiko (see above).
 * Meaningful Name: The Commander's name has something to do with his ability to manipulate time.
 * Meganekko: Megumi Kurogane and Misao Sakimori
 * Multinational Team: Jim Skylark's elite group
 * Musical Assassin: Reiko can use her powers when she's playing music, so...
 * New Era Speech:
 * New Transfer Student: Reiji Kageyama, and Satoka Tachikawa, from GK 21. The gatekeepers, barring Ruriko, also apply, as they all had to transfer to AEGIS's combination school/secret base when they join the team.
 * The Omniscient Council of Vagueness: Comes as either the invader leaders, or the group of AEGIS commanders
 * Panty Shot: In the very last scene of the last episode: as Shun and Ruriko stand together watching a sunset, Ruriko's skirt blows in the wind and the camera twice gets a clear view of white panties.
 * Also the shtick of the "Ooh, intense!" girl. Over the course of no less than five Marilyn Maneuvers, more is visible each time, until we see a close-up of the front followed by her winking at the camera.
 * Paper Thin Disguise: It's inexplicable how no one realized Kageyama was their enemy when all he puts on for disguise is a pair of shades (which he carries around everywhere anyway) and a little unbuttoning of his shirt.
 * Parental Abandonment: Shun's resentment of his dead father; Kaoru, Reiko, Megane, Big Boss and Feiling have no visible parents (Reiko's are divorced); Yukino's parents died some eight hundred years ago. Averted by Ruriko's intact and happy family. Possibly subverted by Megumi's apparent desire that her own parents would just go away and die somewhere.
 * Playing With Fire: Feiling
 * Pointless Band-Aid: Shun
 * Psychotic Smirk:, and.
 * Put On a Bus (Francine, who was one of the main Gatekeepers in the game, was completely omitted in the anime. Likewise Misao's role in the game was replaced with Megumi in the anime (it should be noted that the characters do have different personalities). Both Misao and Jun also played a more important role in the game and were both main characters as opposed to their brief cameos in the anime.
 * Really Seven Hundred Years Old: Yukino
 * Red Herring: The old sweet potato vendor from one episode, who owns an exact duplicate of the Invaders' Sinister Shades. It turns out that the real invader was
 * The Rival: Jim Skylark, who is also The Ace
 * Secret Legacy: Shun learns why his neglectful father was never home.
 * Sexy Secretary: Who also happens to be School Nurse.
 * Shock and Awe: Jun Thunders, a member of Jim's team.
 * Shout Out:  transformation sequence is similar to Ultraman's
 * Sinister Shades: The Invaders wear these, as do the Big Bad.
 * Small Girl Big Gun:
 * The Smart Guy: Megane
 * Spider Sense: Misao Sakimori from Jim's team, who dictates the exact time before something happens, giving the group a serious tactical advantage.
 * Spoiler Opening: The surprise Big Bad gets revealed even before the first episode.
 * Strange Salute: The AEGIS Salute
 * Student Council President: Ruriko
 * Super Prototype: Gate Robot Unit 00, which has no Power Limiter, unlike the later production models.
 * Super Speed: Jim Skylark's Supersonic Gate.
 * Super Strength: Kaoru, who additionally has Super Speed, as her Gate power amplifies her physical abilities.
 * Takehito Koyasu: Bancho
 * Technopath: Kaiser Kikai
 * Television Tie in Novel: Gatekeepers 1985, which bridges the gap between Gate Keepers and GK 21.
 * The Anime of the Game
 * Those Wacky Nazis: Kaiser Kikai has the look, and has a tendency to shout "HEIL VADER!" whenever he shows up.
 * Time Stands Still: The Commander's power. Which is only depicted in the novel.
 * Transformation Trinket: Infected people somehow get Sinister Shades from nowhere, which they put on to transform into Invaders.
 * Unstoppable Rage: Gatekeepers who go into an Unstoppable Rage gain "Inverse Gates", which do the exact opposite of their original powers. This is deconstructed in GK 21
 * Victorious Childhood Friend: Subverted by
 * The Virus: The Invaders in Gate Keepers 21.
 * Waif Fu: Kaoru Konoe and her tank-lifting introduction.
 * The Worf Effect: Kaiser Kikai easily defeats Jim's elite Multinational Team with his tank on a battleship, forcing AEGIS to deploy the Super Prototype.
 * The War Room: Any AEGIS base. AEGIS Japan comes complete with Bridge Bunnies.
 * Zettai Ryouiki: Feiling wears a long red qipao with slits long enough to creat the effect.
 * The War Room: Any AEGIS base. AEGIS Japan comes complete with Bridge Bunnies.
 * Zettai Ryouiki: Feiling wears a long red qipao with slits long enough to creat the effect.