[[quoteright:230:http://static.tvtropes.org/pmwiki/pub/images/230px-Aion_manga_vol_1_cover_1099.jpg]]

Tatsuya Tsugawa's parents die in an accident. One week after the accident, he has inherited millions of yen and is now starting high school. He tries to follow his father's last words about being a man of high caliber. A little while later, Tatsuya tries to rescue a girl, Seine Miyazaki, from being bullied. She coldly rebuffs his attempt and claims that she likes being bullied. He still tries to help her despite the fact she told him to leave her alone. It turns out that Seine is immortal, and she kills parasitic bugs that control humans with a snake named Aion that devours them.

''Hekikai no [=AiON=]'' is a 2008 fantasy and horror manga by Creator/YunaKagesaki, who is also known for ''Manga/SakuraNoIchiban'' and ''Manga/{{Karin}}''. The series is serialised by ''Dragon Age''. The manga finished with 11 volumes in October 14th, 2012. Three volumes of the series was translated in english by TokyoPop under the name ''Aion'' before it eventually closed its North American publishing division.

Not to be confused with the game ''VideoGame/{{Aion}}''.
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!!Tropes this series provides are:
* AdventuresInComaland: Tatsuya went into [[spoiler:Seine's]] induced dream in order to wake her up.
** AllJustADream: according to Shizuki in a ,somehow, [[http://www.mangareader.net/hekikai-no-aion/27/32 spooky way]].
* AllLoveIsUnrequited: Nagisa is in love with Tatsuya who is in love with Seine who is in love with [[ParentalSubstitute Simon]]
* AttemptedRape: Since in Japan, your first kiss is SeriousBusiness, you can considered Tatsuya saw the attempted kiss of Sheila as this.
* ArtEvolution: Compare [[http://www.mangareader.net/1646-52639-36/hekikai-no-aion/chapter-1.html this]] with [[http://www.mangareader.net/hekikai-no-aion/28/28 this]]
* BabiesEverAfter: At the end, Croquet had three puppies.[[spoiler:..and one inherited Chibi Piyo Piyo's powers [[MixAndMatchCritter and wings!]]]]
* BarbieDollAnatomy: The mermaids spend a lot of time topless, but even without their [[GodivaHair hair covering their breasts]], there's only a vague outline of detail at best.
* BettyAndVeronica:
** Played straight with Yoshiyuki as UnluckyChildhoodFriend Betty and Seine as ActionGirl Veronica.
** Then played with; ShrinkingViolet but exotic mermaid and human but BrokenBird witch. Though Tatsuya shows more attraction to Seine, he still can leave Sheila alone when she's in trouble...
* BigBad: The evil mermaids.
** BiggerBad: [[spoiler: Demiurge]]
* TheBigDamnKiss:
** [[spoiler: Tatsuya to Seine in chapter 39]]. Also counts as ForcefulKiss.
** Also, [[spoiler: Seine to Tatsuya]] at the end of chapter 39, beginning of chapter 40; {{Subverted}} because it was just to [[spoiler: take a Mushi out of him]].
** And before of all that, [[spoiler: Sheila to Tatsuya]] at the end of chapter 38.
* BigFancyHouse: Tatsuya's house.
* {{Bishounen}}: 19-years-old Tatsuya. Minato also counts, with or without TimeSkip
* BittersweetEnding: [[spoiler:Tatsuya and Seine get the chance to have a RelationshipUpgrade, the town is saved from being sink, Enati's threat is avoid and Sheila gets to have a genuine mermaid lower body (sacrificing her human legs and return to land); Seine gets to kill 2 of seven mermaids after hearing Simon's history through Ariel and seems to give up her revenge, but after the years, Tatsuya dies for old age and Seine is seeing besides his grave, just as young as always]].
** [[spoiler:But then Tatsuya's soul seems to came for Seine as she gives up to Aion and both are TogetherInDeath, and Aion returns to it's true owner, Sheila, returning her her voice]] making the end a little more sweet but still a TearJerker.
* BloodSplatteredWeddingDress: [[http://www.mangareader.net/1646-52642-27/hekikai-no-aion/chapter-4.html Not a wedding dress but a kimono]]; still the same impact
* BrainwashedAndCrazy: All the victims of the mushis become this when they take a look at Seine's face.
* BreatherEpisode: Chapter 28. It's even called "Renewing Moods"
* ChekhovsGun: Croquet eating Chibi Piyo-piyo.
* ClothingDamage: Deconstructed with [[{{Gorn}} all the wounds Seine got]]
* CovertPervert: Everybody has their thing.
** Yuzuki is a {{Lolicon}}.
** Shizuki likes old men.
** Yoshiyuki gets too excited after seeing [[WholesomeCrossdresser Tatsuya]] in a {{Meido}} costume and a wig
** [[spoiler: Simon]] likes to raise little girls.
** Tatsuya gets too excited after [[spoiler: seeing a 4-year-old and a 8-year-old Seine within her dreams.]]
* CrossOver: With {{Karin}}.
* CruelToBeKind: Seine must force the Mushi's host into hate her and kill her so she can rip out the Mushi.
--> '''Seine:''' "It's fine like this. She should hate me more. If she does, I can save her quickly."
* CycleOfRevenge: Seine, looking for avenge Simon, kills a mermaid in an OffWithHerHead way; later that same mermaid comes [[BackFromTheDead back from the dead]] looking for revenge, staring the cycle all over again
* EnforcedColdWar: [[spoiler: Tatsuya forced Seine and Sheila to live under the same roof when Sheila left the sea]]. No wonder why he ended punched in the face.
* EveryoneCanSeeIt:
** Well...everyone but [[ObliviousToLove Tatsuya]] can see that Yoshiyuki is in love with him. He's just that dense
** Also, everybody starts to see that Tatsuya feels something for Seine
* {{Expy}}:
** Seine looks like Manga/{{Karin}} but with longer, green hair and as an EmotionlessGirl instead of a ShrinkingViolet.
** Yoshiyuki is also this for Maki of the same series.
** Minato looks like a younger, black haired version of Ren.
** Tatsuya kinda looks like a Male!Asami, Asami is from Kagesaki-sensei's Manga/SakuraNoIchiban
* FaceFault: Minato had this in chapter 9.
* FamousLastWords: Tatsuya's father's last words were ''"A man of the Tsugawa Family must be a high caliber man!!"''
* FanserviceWithASmile: PlayedForLaughs, since all the boys dress in {{Meido}} costumes. Tatsuya gets more focus since his skirt is too short because of him growing, and his weeping, red face.
* FirstNameBasis: When Tatsuya at first started calling Seine by her last name. After they started living together, She tells him to call her by her first name since she thinks Miyazaki is "made up". Also, Seine just calls him Tatsuya.
* FirstKiss / SacredFirstKiss: Seine and Minagi got their first kisses taken [[spoiler: by [[{{Lolicon}} Yuzuki]]]]
* {{Foreshadowing}}: Tatsuya [[FirstNameBasis calls Seine "Seine-chan"]], she shivers at this. The reason? One of the twins calls her the same.
* FourGirlEnsemble: Yuzuki (TheDitz), Seine (DeadpanSnarker), Shizuki ([[TheTease The sexy one]]) and Sheila (WildCard).
** And Before Sheila appeared, Nagisa as the WildCard.
* FuroScene: Seine had one in chapter 3. Yoshiyuki had it in chapter 7
* GenderFlip: [[http://img580.imageshack.us/img580/4753/158mf.jpg This]] [[http://img840.imageshack.us/img840/9796/159mt.jpg volume 10's]] [[http://img542.imageshack.us/img542/1755/160m.jpg omake]].
* GiantWallOfWateryDoom: The way the mermaids collet psyche.[[spoiler:..Also, [[http://www.mangareader.net/1646-58635-36/hekikai-no-aion/chapter-26.html Seine's image of death inside her mind]]]].
* GirlInABox: Seine, before moving to Tatsuya's house used to live in a cardboard box!
* {{Gnosticism}}: Implied.
* GoingCommando: Seine spends half of the 5th chapter without panties because they got ruined and she only had a pair.
* {{Gorn}}: Cortesy of BrainwashedAndCrazy people upon Seine. Let's make a list:
** Chapter 2: Thrown off a building
** Chapter 4: Poisoned and stabbed with a glass bottle
** Chapter 8: Choked to death
** Chapter 17: BatterUp to death
** Chapter 19 and 20: ImpaledWithExtremePrejudice
** Chapter 21: Stabbed [[KnifeNut with a knife]] in the lung
** Chapter 22: Disarmed
* GratuitousForeignLanguage: The mermaids (and Shizuki, sometimes) use greek words.
* GrievousBottleyHarm: In chapter 4, Minagi's father stabs Seine with a bottle.
* HearingVoices: As the mushis manifest to the host after seeing Seine.
* InnocentCohabitation: Tatsuya and Seine, who now moved in with Tatsuya.
* InnocentFanserviceGirl:
** Seine, being an EmotionlessGirl who consideres herself as a monster, is not bothered if somebody sees her shirtless or [[ThanksForTheMammary grabs her chest]] and she even takes her clothes up once to show Tatsuya that her wounds are healed. Of course, even if she doesn't freak out, Tatsuya does.
** Sheila counts too, [[spoiler: being a mermaid, she is shirtless all the time; every time she sees Tatsuya after getting her human form, she's naked]] and of course Tatsuya gives her the classic PleasePutSomeClothesOn scream.
* ImagineSpot:
** Tsukasa, every time someone says they are living with Tatsuya because he picks them up, he has a imagine spot of them as cute, pitiful homeless [[AlwaysFemale girls]] under the snow.
** Tatsuya also has one of Seine in [[http://www.mangareader.net/1646-52649-12/hekikai-no-aion/chapter-11.html a cute two-piece swimsuit]] after hearing they're going to the pool [[ImagineSpotting and Yuzuki seems to be watching it.]]
* {{Jerkass}}: All the mermaids except Ariel.
* LaserGuidedAmnesia: Everybody who had a Mushi taken off by Seine doesn't remember anything about their hatred towards her [[spoiler: as long they get to kill her before The Mushi was taken off]].
* {{Lolicon}}:
** Yuzuki is acknowledged as this, PlayedForLaughs and LesYay, of course.
** [[spoiler: Also, Tatsuya may be this, with a little taste of SingleTargetSexuality, upon Seine. He even [[DopeSlap punches himself]] after Chibi Seine called him "Onii-chan" to keep his [[CovertPervert "True personality"]] in line]]
* LoveTriangle: Sheila -> Tatsuya -> Seine, [[TriangRelations making it type 5]]
* LuminescentBlush: Tatsuya is prone to this. Also Yoshiyuki, Sheila, heck, even [[TheStoic Seine]]
* MayDecemberRomance: Tatsuya's parents had an age loop of 30 years and still they were HappilyMarried
** MayflyDecemberRomance: Tatsuya one-sided crush on Seine.
* MeetCute:
** Tatsuya first saw Seine after she steps on his money when a couple of bullies throw it to the ground.
** Also, Tatsuya's parents. They met when he saw her hanging white radishes.
* MermaidProblem: PlayedForDrama...but in a ''very'' [[http://www.mangareader.net/hekikai-no-aion/31/9 hilarious]] [[http://www.mangareader.net/hekikai-no-aion/31/10 way]]
* MindControlDevice: The Mushis.
* ModestyTowel: As [[http://www.mangareader.net/1646-57244-14/hekikai-no-aion/chapter-24.html improvised bikini]].
* MoreThanMindControl: Mushi infected [[spoiler: Nagisa]]. Also Himenuma; though Mushi infected, he seems to have a crush on Sheila.
* NoPeriodsPeriod: One day, Yoshiyuki was all mad, Minato jokes that it must be "that day of the month".
* NotWhatItLooksLike: It has moments.
* {{Omake}}: There're lots of this in each volume, where Kagesaki talks about ups and downs of her while making Aion.
* OminousFog: Enati, the third mermaid can use fog to [[spoiler: make an illusion of Simon]].
* OurMermaidsAreDifferent: The evil mermaids.
* PantyShot: Seine had this in the first chapter.
* PetTheDog: Seine comfort Tatsuya in the first chapter saying him he can become a high caliber man
* PervertedSniffing: Sheila smells Tatsuya's necktie.
* PinkySwear: Tatsuya and Sheila do a pinky swear when Tatsuya promises not to tell anyone that she's a mermaid.
* PoolEpisode: It gives us a great reveal...
* PowerTrio: Seine, Shizuki and Yuzuki.
** FreudianTrio: Shizuki (manipulative Superego), Yuzuki (kind Ego) and Seine (vengeful Id).
*** Alternatively, Seine (Superego), Tatsuya (Ego) and Sheila (Id).
** TheHecateSisters: Yuzuki (ditzy maiden), Shizuki (eccentric mother), Seine (bitter crone).
* PrettyFreeloaders: Seine, Shizuki and Yuzuki (for a while) and, after TimeSkip, [[spoiler: Sheila]]. Tatsuya, apparently, has a thing for picking up girls to live in his house.
* RedHerring: The deal with Tatsuya's mother. [[spoiler:Her name was Umi (which means "sea") and Sheila was very much a carbon copy of her image, so many readers thought Umi was a mermaid too. In reality Umi was just a normal human but her psyque was took by Sheila after her death so Tatsuya sometimes could "feel" his mother's scence in Sheila.]]
* TheReveal: Why Tatsuya saw Sheila as his mother at one point? [[spoiler: Because Sheila took her psyche when she died]].
* ReturningTheHandkerchief: Tatsuya used this as an excuse to talk to Seine at the first chapter
* RidiculouslyCuteCritter: [[http://mangafox.me/manga/hekikai_no_aion/v06/c024/31.html Chibi Piyo-Piyo]]
* RuleOfThree: Seine can die just three time per day. If she dies more than that, Aion has to "recharge" itself [[spoiler: and Seine will feel the pain of all those deaths]] before she can use Aion again.
* RunningGag / CaptainObvious: "Earthshake. There's been a lot of them these days."
* SeizaSquirm:
** In chapter 30, Tatsuya and Seine do this while talking to Tsukasa for midnight sneaking out of the house.
** Also, Seine had cramps for sitting so long in seiza at Tatsuya's parents's funeral event.
* ShipperOnDeck: Minagi seems to like shipping Seine with Tatsuya. It helps that it's obvious the guy is really in love with the girl and she wants to help him.
* ShipTease: Seine and Tatsuya.
* ShownTheirWork: There was really a earthquake at Port Royal that sank 2/3 of the city into the sea.
** Also, the mermaids can cause earthshakes from the sea; it's geologically possible that earthshakes provoce volcanic eruptions, therefore the mermaids did cause the Vesuvio eruption.
* SiblingRivalry: All the mermaids are sisters and some of them are trying to overpower her oldest sisters.
* SwallowedWhole: The only way to rip off the mushi from the host's body is Aion swallowing the host to "eat" the mushi.
* ThereIsOnlyOneBed: Shizuki and Yuzuki slept in the same bed Tsukasa was sleeping after getting hit on his head because how "High status" women like them would sleep on the floor?
* TimeSkip: Volume 8 starts 4 years since the events of volume 7. Everybody grows older...[[NotAllowedToGrowUp except Seine]] who's immortal of course.
* TooKinkyToTorture:
** At the first chapter, Minato warns Tatsuya about not messing around with Seine because he thinks she is a masochist who likes being bullied.
*** Hilariously "enforced" by Seine to keep Tatsuya off her neck.
** [[spoiler: After getting ImpaledWithExtremePrejudice and removing one of the long stakes with Tatsuya's help, she had a LuminescentBlush and a dazed expression on her face. [[DoesThisRemindYouOfAnything "A lot of endorphins were released".]] The position they had to take in order to get the stake out (him behind her, pushing her forward towards the wall that the end of the skewer was braced against) was more than a little suggestive to begin with...]]
* TrueLovesKiss: Referenced [[http://mangafox.me/manga/hekikai_no_aion/v06/c024/30.html here]]. Tatsuya's face is [[CrowningMomentOfFunny priceless]].
* WaterfallPuke: [[LethalChef Tatsuya's food]] had this reaction on all people, except Seine. Shizuki is an UpToEleven case, it causes to her puke BloodFromTheMouth.
* WaterWorks: A lot of people cry in this manga but Tatsuya is more prone to it.
* WhamEpisode: Volume 10. ''The whole Volume 10''. [[spoiler:Tatsuya shouts aloud he likes Seine, Sheila infects Tatsuya via stealing his first kiss and send him to kill Seine at her sisters's demand, Tatsuya doesn't kill Seine and instead pulls himself out the friend zone he has been for 4 years by kissing her, Sheila is returned to the sea by Ariel, Shizuki and Yuzuki came back, the fated day is closer...[[ArsonMurderAndJaywalking and Seine is dressed as a]] {{Meido}}]]
* WhatIsThisThingYouCallLove: Seems to be the mermaids's case; specifically the emotional part of "love" since they do know "love" is used by humans to mate.
* YouGottaHaveBlueHair:
** Seine, who has green hair.
** Yoshiyuki, who has purple hair.
* YourApprovalFillsMeWithShame: Seine gets no other option but to rip the Mushis out of a crowd of {{brainwashed and crazy}} students without "exciting" the Mushis first. Shizuki applauds this act. Seine is not amused, since she knows what doing that means. [[spoiler:Hosts who aren't able to satisfy the Mushi's urge to kill Seine find themselves ''still wanting to kill her, but with no idea why''. This isn't good for their sanity.]]
* ZettaiRyouiki: Seine wears rank B.
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