Gate Keepers 21

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.

Gate Keepers 21 contains examples of:

 * All There in the Manual: Bad news: You have 31 years of catching up to do between Gate Keepers and this. Good news: There are some novels and a video game bridging the gap. More bad news: None of them are in English.
 * 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...
 * A Badass Longcoat.
 * Berserk Button: Ayane really likes birds. God help you if you do anything bad to them. Just ask.
 * Better Than It Sounds Anime
 * Blast Out: Yukino and Ghost Girl move from a Mexican Standoff to this in the final episode.
 * Blow You Away: Ayane's Gate power
 * BFS: Satoka's Gate Power calls forth these.
 * Bounty Hunter: 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.
 * Buddy Cop Show: Ayane and Miu (Student and Master Team variation).
 * Continuity Nod: When Ayane and Miyu launch the UP-15 from the self-destructing AEGIS base, Yukino does the "Ooh, intense!" thing that accompanied AEGIS vehicle launches in the original series. No Panty Shots, though.
 * Cool Shades: Kageyama
 * Darker and Edgier: In Gate Keepers, you had hope the The Power of Love would prevail. Here, not so much.
 * Dark Is Not Evil: Kageyama, though he also happens to have elemental light as a power, so...
 * Demonic Invaders: The Invaders, natch.
 * Demonic Possession: Any non-Gatekeeper humans who succumb to any dark emotion open themselves to possession by the Invaders.
 * Disappeared Dad: Shun
 * The Ditz: Miyu
 * Dude, She's Like, in a Coma: Ayane Isuzu is kissed while comatose by Big Bad Stalker with a Crush Yuurei Shoujo, while the former's boss and partner watch helplessly.
 * The End of the World as We Know It: Ghost Girl causes this in the Grand Finale.
 * Equivalent Exchange: Gates run on this. Kageyama points out the fact that most Gatekeepers have suffered some kind of trauma earlier in life. Furthermore,
 * Everybody's Dead, Dave: Almost the entire cast from Gate Keepers die (or are simply written out) in the backstory.
 * Everything's Better with Sparkles: Miu's Gate of Leap gives off quite a lot of them.
 * Gratuitous English: "This is your power. It's a miracle!"
 * GAAAYY-TOOOOHH OHHH-PUUUNN!!!
 * Hammerspace: Where Satoka keeps all her Pure Energy Cool Swords.
 * Hot Chick with a Sword: Satoka, depending on your taste, is a hot chick with an arsenal of swords.
 * Improbably-Female Cast: The only male characters are a Supporting Leader, the two Dragons, and an Inspector Javert who doesn't seem to serve any purpose other than Padding.
 * In a Single Bound: Miu's Gate Power is called Flight but is really more of this trope.
 * Magical Girl: played pretty straight with the Gatekeepers, but also parodied by a "film-in-film" Miu and her friends watch.
 * Meganekko: Ayane
 * Mood Whiplash: 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.
 * Number One Dime: Ayane's bell. Depending on how her surname is spelled in Kanji, could veer into Meaningful Name territory, as one spelling of "Isuzu" means 'Dependable Bell'.
 * Oddly-Named Sequel 2: Electric Boogaloo
 * Older and Wiser: Several characters from the original Gate Keepers, now grown up, act as commanders, mentors, and parents to the new cast.
 * One-Woman Wail: The opening theme.
 * Our Hero Is Dead: At the end of episode 4 of GK 21,
 * Parental Abandonment
 * Plant Aliens: Count Akuma has now become this
 * 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.
 * Psycho Lesbian: Yurei Shoujo, the Big Bad, totally has it in for Ayane. See Dude, She's Like, in a Coma above.
 * Really Seven Hundred Years Old: Yukino and Yurei Shoujo.
 * Reset Button:
 * Sailor Fuku: Miu brings Yukino a seeraafuku for a school uniform. Why not get her an actual uniform of Miu's school? Well, because.
 * Scary Shiny Glasses: Ayane Isuzu.
 * Shout-Out: The crane game has characters from Parappa the Rapper, Azumanga Daioh, and Keroro Gunsou, among others.
 * Spider Sense: 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.
 * The Stoic: Ayane
 * Strong Family Resemblance: Ayane bears a resemblance to her aunt Saemi, Shun's younger sister, except she wears glasses. Saemi furthermore resembles her mother after growing up.
 * Those Two Guys: Miyu's friends, Chinami and Nao-chan.
 * Unknown Rival: Tachikawa Satoka spends most of her free time talking to/critiquing/lecturing Badass Bookworm Isuzu Ayane only to get ignored, or ditched when her back is turned, much to her chagrin.
 * The Virus: The Invaders.
 * Vapor Wear: Yuurei Shoujo.
 * Vapor Wear: Yuurei Shoujo.