* ExecutiveMeddling: Hekikai was originally thought of before ''Manga/{{Karin}}''. Kagesaki-sensei planned it while she was in middle school but had to change her original idea of Hekikai because the executive didn't like it.
* IronWoobie: Seine after killing her first mermaid; she stopped the crying for Simon's death and became an ActionGirl BrokenBird and since then who knows how many things she has took with a [[StoicWoobie stoic face]].
* HollywoodPudgy: Nagisa apparently consideres herself fat and she has taken summer diets that ultimately fail, but she isn't more fat than the rest of the cast.
* HoYay: Tsukasa was meant to be gay for Tatsuya or at least was pointed to be, as seen in this volume 2's [[http://mangafox.me/manga/hekikai_no_aion/v02/c008.5/2.html omake]]. Though, now he's a wholesome fraternal figure to him, some can't stop see the HoYay between them. Even Minagi point it once!
* {{Moe}}: Sheila. {{Woobie}}? Check. CuteMute? Check. A little bit of lost and you can't avoid want to protect her? Damn check.
* TheyWastedAPerfectlyGoodPlot: The whole Shizuki's "fishing something more big than mermaids" talking about Demiurge, her identity as [[spoiler:the goddess Pistis Sophia]] and the past of the twins; not to mention maybe a more epic battle between Seine and all the mermaids; and also the thing about if the death of Tatsuya's parents was really an accident or a murder. It's possible Kagesaki-sensei had more history in mind but Hekikai was victim again of ExecutiveMeddling.
* UglyCute: Croquet, Tatsuya's dog.
* ViewerGenderConfusion: At first look, it's difficult to know that Bella is really a female mermaid.
* TheWoobie:
** Sheila. Her sisters are complete {{Jerkass}} towards her.
** Young Seine counts too. Her childhood before Simon was crap.