* HollywoodPudgy: Nagisa apparently considers herself fat and she has taken summer diets that ultimately fail, but she isn't any 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 volume 2's omake. Though, now he's a wholesome fraternal figure to him, some can't stop seeing the HoYay between them. Even Minagi pointed it out once!
* 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 taken with a [[StoicWoobie stoic face]].
* {{Moe}}: Sheila. {{Woobie}}? Check. CuteMute? Check. A little bit of lost and you can't avoid wanting to protect her? Damn check.
* TheyWastedAPerfectlyGoodPlot: Shizuki's war against Demiurge, her identity as [[spoiler:the goddess Pistis Sophia]] and the twins' past, as well as the question of whether Tatsuya's parents' deaths were really an accident or murder.
* 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}}es towards her.
** Young Seine counts too. Her childhood before Simon was crap.