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 now 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 Yuna Kagesaki, who is also known for Sakura no Ichiban! and 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 Tokyo Pop under the name Aion before it eventually closed its North American publishing division.Not to be confused with the game Aion.
Bittersweet Ending: Tatsuya and Seine get the chance to have a Relationship Upgrade, 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.
But then Tatsuya's soul seems to came for Seine as she gives up to Aion and both are Together in Death, and Aion returns to it's true owner, Sheila, returning her her voice making the end a little more sweet but still a Tear Jerker.
First Name Basis: 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.
Seine, being an Emotionless Girl who consideres herself as a monster, is not bothered if somebody sees her shirtless or 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, 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 Please Put Some Clothes On scream.
Freudian Trio: Shizuki (manipulative Superego), Yuzuki (kind Ego) and Seine (vengeful Id).
Alternatively, Seine (Superego), Tatsuya (Ego) and Sheila (Id).
The Hecate Sisters: Yuzuki (ditzy maiden), Shizuki (eccentric mother), Seine (bitter crone).
Pretty Freeloaders: Seine, Shizuki and Yuzuki (for a while) and, after Time Skip, Sheila. Tatsuya, apparently, has a thing for picking up girls to live in his house.
Red Herring: The deal with Tatsuya's mother. 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.
The Reveal: Why Tatsuya saw Sheila as his mother at one point? Because Sheila took her psyche when she died.
Water Works: A lot of people cry in this manga but Tatsuya is more prone to it.
Wham Episode: Volume 10. The whole Volume 10. 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...and Seine is dressed as aMeido
Your Approval Fills Me with Shame: 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. 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.