The characters of ''Manga/HekikaiNoAiON'' include:

!!Seine Miyazaki

Seine Miyazaki is a 300 year old girl with an immortal body who Tatsuya invites to live in his home; before that she lived in a cardboard box in a park. In the first chapter, Seine looks like a masochist because she seems to like being bullied. Actually, she becomes close to people who are infected by "mushis", a kind of parasitic, mind-controlling bugs, in order to hunt them. Her hunting ways go against the kind and pacifist Tatsuya's beliefs. Her motive is to avenge her foster father, Simon's death by killing the people of sea, the mermaids. She controls a black snake called Aion that lives inside her. Her weakness is her inability to swim.

* AbusiveParents: [[spoiler:Her second father abused and traumatised her to the the point she is unable to swim.]]
* ActionGirl
* [[spoiler:AdventuresInComaland]]
* AnimalMotifs: Cats. So much that she portrayed as a CatGirl in an [[ omake]].
* BerserkButton: Shizuki and Yuzuki. Also anything mermaid-related.
* BrokenBird: [[spoiler:The first 4 years of her life were horrible, her ParentalSubstitute was killed in front of her and she's seen lots of people she cared about dying as well.]]
* {{Delinquent}}: She's seen as this by Tsukasa.
* EveryoneCallsHimBarkeep: Seine is frequently called "witch" instead of her real name.
* {{Expy}}: Of ''Manga/{{Karin}}'', though only in appearance.
* GirlInABox: Lived in a cardboard box prior to meeting Tatsuya.
* Creator/HarukaTomatsu: Her Drama CD VA.
* HugeGuyTinyGirl: With 19-year-old Tatsuya.
* IAmAMonster
* IGaveMyWord: When she finds someone who [[PleaseDontLeaveMe asks her to stay by his/her side,]] she stays until they die, tell her to leave, or obstruct her revenge, [[spoiler:because she used to be completely alone, too.]]
* {{Immortality}}
** TheAgeless
** GoodThingYouCanHeal
** ImmortalLifeIsCheap
** ResurrectiveImmortality
* ImportantHaircut: She used to have a mild example of RapunzelHair, but she cut it when she ultimately decided to follow the YouKilledMyFather path.
* InnocentFanserviceGirl: More like "Indifferent Fanservice Girl".
* IWorkAlone
* LeParkour: With Aion's help. Tatsuya even compares her to a roof-jumping cat.
* {{Neet}}: After the TimeSkip.
* NotGrowingUpSucks: Seine's had to go through high school many, many times to maintain cover. She even complains about it while talking about her nth school festival.
* NotSoStoic
* OrdinaryHighSchoolStudent: Subverted.
* ParentalAbandonment: [[spoiler:She doesn't know anything about her biological parents.]]
* PowerTattoo
* PrettyFreeloader
* [[Really700YearsOld Really 300 Years Old]]
* [[spoiler:RelationshipUpgrade[=/=]OfficialCouple]]: With Tatsuya.
** [[spoiler:TogetherInDeath]]
* SugarAndIcePersonality
** Later, she evolved to a {{Tsundere}} towards Tatsuya.
* SuperDrowningSkills: Seine can't swim. Even in a kid's pool with water-wings, she sinks like a rock. And she fights the people of the sea. The irony.
* TeensAreShort: Her height is 1.46m (4' 7").
* TheNoseKnows: She claims infected humans have a different smell than normal humans, and she's learned how to distinguish it over time.
* WhenSheSmiles: [[ Just look at her]]. [[DoggedNiceGuy Tatsuya]] is out of breath.
* [[YouGottaHaveBlueHair You Gotta Have Green Hair]]
* YouKilledMyFather: Her {{Revenge}} motivation.
* ZettaiRyouiki: Mostly Grade B, sometimes Grade A.

!!Tatsuya Tsugawa

After the death of his parents in a car accident, Tatsuya tries to fulfill his father's last wish of becoming a "high caliber" man. After "saving" Seine from bullying, he insists on trying to help her, even after finding out her real goals. He is kind, optimistic, and believes in other people's good side, which is sometimes an obstacle in Seine's revenge. He is a bad cook, but somehow Seine is not bothered by his food. He has a crush on Seine and wants to be acknowledged as a "high caliber" man by her.

[[TimeSkip Four years later]], he starts going to University at the city.

* AccidentalPervert: Luckily for him, the girls are an IceQueen and a FishOutOfWater.
* {{Adorkable}}
* AlliterativeName
* CurtainsMatchTheWindow
* BecauseYouWereNiceToMe: After Seine [[PetTheDog encouraged him]] by saying he can become a "high caliber" man, he fell for her and wanted to be acknowledged by her.
* TheBigDamnKiss: [[spoiler:Finally the guy gathers the courage to give one to Seine in chapter 39]].
* {{Bishounen}}: First the cute kid edition, then later the hot guy edition.
** IGotBigger: Though 19-years-old Tatsuya looks quite the same as 16-years-old Tat-chan, just bigger.
* BreakTheCutie: Let's just say chapter 15 wasn't very nice to him. He gets better at the end of chapter 16 with Seine's help.
%%* CrossdressingVoices
* DoggedNiceGuy: To Seine.
** [[spoiler:RelationshipUpgrade[=/=]OfficialCouple]]
* DraggedIntoDrag: Forced to dress up as a {{meido}} as part of his class's cafe staff. And the girls rejoiced.
** AttractiveBentGender: ...but no one can deny he looks so cute as a female.
* HeroicBSOD: At his parents' funeral. [[JustifiedTrope Understandable]].
* HeroicWillpower[=/=]ThePowerOfLove: Why he didn't [[spoiler:kill Seine even after being infected.]]
* InTouchWithHisFeminineSide: He tries to take care of his house once since his uncle left--make meals, clean, and so on. Minato jokes that he acts like a wife.
* [[spoiler:JourneyToTheCenterOfTheMind]]
* LethalChef: As [[IllGirl Shizuki]] [[BloodFromTheMouth discovered]]. Only Seine is not bothered by his cooking.
* LivingEmotionalCrutch: He became more or less this to Seine after [[spoiler:his JourneyToTheCenterOfTheMind of Seine and learning why she's so broken]]. He promised her to always be there for her and, as a bonus, he reminds her of a past friend.
** KidWithTheLeash: He's also the only one able to control and stop Seine when she's on a rampage. [[spoiler:Also, he could be this to Sheila too.]]
* LonelyRichKid: Until Seine promises him to stay.
* NonActionGuy: Though he's helpful in other ways.
* ObliviousToLove: He can't realize Nagisa has a crush on him since they were around 12 until Minato told it to his face and was horrified that even Seine could read her behavior better than him in only one month.
* OneHeadTaller -> HugeGuyTinyGirl: With Seine.
* OrdinaryHighSchoolStudent
* ParentalAbandonment: [[MyParentsAreDead His parents died]] a week before the start of the story.
** ConvenientlyAnOrphan: Admit it, Tatsuya suddenly filling his house with a bunch of young women will be very awkward if his parent were alive.
* SayMyName: "MIYAZAKI-SAN!"
** After FirstNameBasis: "SEINE-CHAN!"
* SecretKeeper: He promises to keep the secret that Sheila is actually a mermaid after doing a PinkySwear with her.
* SingleTargetSexuality: Seine.
* TeensAreShort: 1.62m (5' 4") at the age of 16.
* [[spoiler:TogetherInDeath]]: With Seine.
* YouAreNotAlone: To Seine.

!!Nagisa Yoshiyuki

Nagisa is a girl in Tatsuya's class, a long-time acquaintance, and would probably like to get closer to Tatsuya, but ever since Tatsuya found a lost puppy (soon to be Crokette), Nagisa has had violent allergic reactions whenever she is in Tatsuya's house. So she simply stays away from him, even though she loves him.

[[TimeSkip Four years later]] she works as [[FanserviceWithASmile a waitress]] in a cafe.

* BerserkerTears / BrokenTears: Cried these while [[BrainwashedAndCrazy choking Seine to death]].
* CanNotSpitItOut: She just can't say: "Tatsuya, I love you."
* ControlFreak: Mild example. Since she is in charge or her house, she's very strict even with her [[BumblingDad father]].
* DamselInDistress: [[spoiler:When she was infected by Mushis]].
* {{Expy}}: Of Maki from ''Manga/{{Karin}}''. They have a similar appearance to each other and are childhood friends to the main character. Nagisa's BelligerentSexualTension with Minato also parallels to Maki and Yuuji's SlapSlapKiss relationship.
* ExpositoryHairstyleChange: After the TimeSkip, she's seen with a bob instead of her long side ponytail.
* FanserviceWithASmile: She works as a waitress in a cafe.
** {{Meido}}: The cafe's uniform.
* FirstNameBasis: With Minato. She also used to have this with Tatsuya, but since everybody thought she was [[{{Golddigger}} taking advantage of him for being rich]] and her allergies kicked in, she's starting to keep her distance.
** They get back to the first-name basis after [[spoiler:the Mushi incident]].
* HeroicSelfDeprecation: She thinks she's a bad, selfish person, unworthy of MessianicArchetype Tatsuya. She gets better.
* LikeAnOldMarriedCouple / ShipTease / BelligerentSexualTension: With Minato.
* MissingMom
* MoodSwinger
* MoreThanMindControl: [[spoiler:When she was infected by Mushis,]] they used her jealousy of Seine and her fear that Tatsuya knows she wanted his dog dead to break her will.
* PlotAllergy: She's allergic to dog hair. After Tatsuya decided to adopt Croquett, she started to avoid going at his house so he doesn't see her sneezing. All this make her hate the dog that would become Tatsuya's lasting family, droving her into more self-hate and making easier [[spoiler:her infection]].
* SingleWomanSeeksGoodMan: She fell for Tatsuya when he defended her from accusations of being a gold digger.
* UnluckyChildhoodFriend
* [[YouGottaHaveBlueHair You Gotta Have Purple Hair]]

!! Minato Higuchi

Tatsuya's friend and Nagisa's neighbor. More worldly and savvy than his friend, he notices that Seine is not a normal girl.

* {{Bishounen}}
* TheConfidant: He's the only who Nagisa can talk with about her crush on Tatsuya and her allergy to dog hair.
** KeepingSecretsSucks
* DisappearedDad
* {{Expy}}: Of [[Manga/{{Karin}} Ren]].
* {{Foil}}: To Tatsuya.
** BromanticFoil
* LikeAnOldMarriedCouple / ShipTease / BelligerentSexualTension: With Nagisa.
* LongHairedPrettyBoy: After TimeSkip.
* MuggleBestFriend
* PermaStubble: After TimeSkip.
* SensitiveGuyAndManlyMan: Manly man to Tatsuya's sensitive guy.
* TallDarkAndHandsome
** TallDarkAndSnarky: To some degree.
* ThoseTwoGuys: With Nagisa.

!! Minagi Shyyuusuke

Tatsuya's little second cousin.

* BoundAndGagged: Her own parents did this to her when they were {{brainwashed and crazy}}.
* CanineCompanion: After Croquett helped her, she started to have a soft spot for him.
* CheerfulChild
* ChildrenAreInnocent:...but she can tell that Tatsuya's house is full of perverts.
* CoolBigSis: She sees Seine as this after TimeSkip.
* CurtainsMatchTheWindow
* TheCutie
** BreakTheCutie: Chapter 3 and 4 did this to her, but thanks to LaserGuidedAmnesia as ResetButton, it was like this never happened.
* GirlishPigtails
* HairColorDissonance: Is she a brunette, dark blonde, or even a strawberry blonde?
* HilariouslyAbusiveChildhood: Her father wants to pay off his debts by [[KissingCousins marrying her to Tatsuya]] and getting at Tatsuya's inheritance; she declares [[ParentalNeglect her parents barely pay attention to her]] and just worry about money, and [[ParentsAsPeople when they try to care, they just shame her]]. All PlayedForLaughs.
* [[spoiler:IGotBigger]]
* MsExposition: She explained the formation of fog, which was a plot point in chapter 31.
* OutOfFocus: {{Lampshaded}} in chapter 25. [[spoiler:This may change now that she lives in Tatsuya's house]].
* ShipperOnDeck: To Seine x Tatsuya.
* SpoiledBrat: She grows out of it after [[spoiler:the Mushi incident with her parents]].
** SpoiledSweet
* ThirdPersonPerson
* TokenMiniMoe: And first victim of Yuzuki's ForcefulKiss.[[ least this time]].
* WiseBeyondHerYears: She's the most reliable one in [[spoiler:Tatsuya's house]].
--> '''Minagi:''' With [[HilariouslyAbusiveChildhood that kind of parents]] around you, you'll grow up to be reliable even if you don't want to.
* YaoiFangirl: She may be joking, though.

!! Tsukasa Kaimaki

A protege of Tatsuya's father and Tsugawa's family lawyer in charge of Tatsuya's money until he gets old enough. He and Tatsuya have a fraternal relationship, even though they are not blood relatives. He cares and looks after Tatsuya but his job as a lawyer prevents him from being able to be close to him.

* BigBrotherMentor: To Tatsuya; who calls him "Tsukasa-[[UsefulNotes/JapaneseSiblingTerminology niisan]]".
* {{Bishounen}}
** HelloAttorney
* CloserToEarth: He says to Tatsuya that he can't go around picking girls to live in his house. [[RealityEnsues And he's right]].
* LikeBrotherAndSister: Non-romantic way. He has no blood tie with Tatsuya or Minagi, but he's a member of the family.
* [[spoiler:LikesOlderWomen: He had a crush on Tatsuya's mother, Umi, when he was a teenager]].
* [[spoiler:LovingAShadow: He seems to have a crush on Sheila because she looks like a carbon copy of Umi]].
* {{Megane}}
** ScaryShinyGlasses: When pissed off.
* MenDontCry: {{Averted}}. He breaks down every time Tatsuya acts as a delinquent on his eyes.
* {{Muggle}}
* NoIndoorVoice: Most of his dialogue is yelling at Seine or Tatsuya, usually while crying.
* NoRespectGuy: To Seine.
* OlderThanHeLooks: He looks like in his early 20, but it's 27. After the TimeSkip, he looks practically the same but now he's ''31''.
* SharpDressedMan
* TallDarkAndHandsome
* TeamChef: After TimeSkip. He became this after moving to Tatsuya's house [[spoiler:since Yuzuki was PutOnABus along with Shizuki]].


Shizuki is the older sister of the "Akuma Twins". Being the older, she is sarcastic, lazy and smart but she has a weak body; she can get sick and tired easily, she cannot swim in pools because of the chlorine, and Tatsuya's food almost killed her (but then, he ''is'' a LethalChef). She has an immortal body and has lived for centuries. She gave immortal bodies to Seine and Simon. Her former name was "Pistis Sophia" but she changed because it was "stupid". She is a Oyaji-con (she is attracted to mature, handsome men).


* AffectionateGestureToTheHead: To Yuzuki. Also sometimes to Seine, just to annoy her.
* AnimalEyeSpy: Shizuki can see through the eyes of Chibi Piyo-piyo. [[spoiler:and since Croquet eat Chibi Piyo-piyo, she can see what Croquette sees.]]
* ButNowIMustGo: [[spoiler:Now that Seine accomplished her revenge for killing Ariel, she and Yuzuki leave her at Shiomigahama to search for a new person to hunt mermaids]].
* CovertPervert: She's an "Oyaji-con"; she likes [[GrandPaWhatMassiveHotnessYouHave old, handsome men]]
* [[spoiler:[[GodInHumanForm Goddess in Human Form]] / [[PhysicalGod Physical Goddess]]]]
* IdenticalTwinIDTag: Shizuki is the one with the [[LittleBlackDress Long Black Dress]]
* IcyBlueEyes
* IllGirl: Her body is so weak that she can't swim in pools because of the chlorine, gets tired easily, and Tatsuya's food [[BloodFromTheMouth almost killed her]]. Luckily she's immortal.
* ImpossibleHourglassFigure
* ManipulativeBitch
* MeaningfulName: Shizuki means "calm moon".
** [[spoiler:Also, her former name. "Pistis Sophia" is the female divinity of gnosticism that means "Faithful Wisdom"]].
* MissionControl
* MulticoloredHair: Black, with blonde bangs.
* {{Neet}}:She does absolutely nothing but eat and sleep. Partially justified as she's an IllGirl.
* NoblewomansLaugh
* OjouRinglets
* OneeSama: to Yuzuki.
* PointyEars: Probably due to being a demon.
* PolarOppositeTwins: With Yuzuki.
* PrettyFreeloader
* [[spoiler:PutOnABus]]
** [[spoiler:TheBusCameBack]]: In chapter 40.
* Really700YearsOld
* ShowSomeLeg: In Volume 4's index.
* SpotOfTea
* ThemeTwinNaming: With Yuzuki.
* TsurimeEyes


Yuzuki is Shizuki's younger sister. She is the polar opposite of her sister: she has superhuman strength, is diligent and a good cook, but is a little dumb. She is good-hearted and carries out her Onee-sama's orders. She also has an immortal body and has lived for centuries. She is a ComedicLolicon (she gets excited by children, especially girls) to the point of kissing every child she meets on the mouth (occasionally ''with tongue'') upon first sight.

* ApologeticAttacker: When "attacking" Seine at her sister's demand.
* BigDamnHeroes: She saves an exhausted Seine from Bella's attack.
* BoobsOfSteel: The physically strongest character in the series and one of the bustiest.
** LightningBruiser
* BridalCarry: Sometimes she carries Shizuki around like this.
* ButNowIMustGo: [[spoiler:Now that Seine accomplished her revenge for killing Ariel, she and Shizuki leave her at Shiomigahama to search for a new person to hunt mermaids]].
* CombatStilettos
* TheDitz: Shizuki even said that Yuzuki has the mind of a kindergartener.
* DumbMuscle
* ExtremeDoormat / TheCaretaker: To her sister.
* ForcefulKiss: The way she expresses her love for little girls. '''Any''' little girl.
* [[spoiler:[[GodInHumanForm Goddess in Human Form]] / [[PhysicalGod Physical Goddess]]]]
* IdenticalTwinIDTag: She's the one with the LittleBlackDress [[MiniDressOfPower Of Power]].
* ImpossibleHourglassFigure
* ImTakingHerHomeWithMe: Yuzuki seems to have this sort of reaction to every small, cute child she sees.
* InnocentBlueEyes
* MsFanservice:
** Bikini in the PoolEpisode.
** Her {{Meido}} outfit at "work".
* MulticoloredHair: Black with blonde bangs.
* NoSenseOfDirection: She gets lost inside a school, mostly because of her ditziness.
* TheOjou
* MeaningfulName: Yuzuki means "kind moon".
* {{Meido}}: She works as a maid in Tatsuya's house.
** GiantPoofySleeves
** OfCorsetsSexy
** ZettaiRyouki
*** StockingFillerRinglets
* PolarOppositeTwins: With Shizuki.
* PointyEars: Probably due to being a demon.
* PrettyFreeloader: But at least she cleans and cooks.
* [[spoiler:PutOnABus]]
** [[spoiler:TheBusCameBack]]: In chapter 40.
* Really700YearsOld
* SupremeChef / TeamChef
* TaremeEyes
* ThemeTwinNaming: With Shizuki.


She was introduced as a new transfer student, and Tatsuya is given the assignment of teaching her sign language. [[spoiler:The youngest of the mermaids, she is despised by all her older sisters except for Ariel. Seine thought she had killed her some 300 years before, but Ariel restored her body almost entirely, leaving her badly scarred about the neck and unable to talk, but with the ability to change herself to have legs and walk about on land. Her true goal is infect a lot of people to order to take revenge on Seine for leaving her without her voice.]] She falls in love with Tatsuya.

[[TimeSkip Four years later]] she meets Tatsuya as he convinces her [[spoiler:to stop thinking of revenge]] but [[spoiler:she's discovered by two of her sisters and leaves the sea in fear. Tatsuya takes her into his house even though Seine is there; Seine allows her to live, but as a hostage against her sisters]]. She seems very jealous of Seine for being the target of Tatsuya's affections.

* [[spoiler:AntiVillain: [[SlidingScaleOfAntiVillains Type 2]] ]]
** After 4 years, [[spoiler:she evolved to [[SlidingScaleOfAntiVillains Type 4]]]] since she still hates Seine.
* AnguishedDeclarationOfLove: And with HandSignals.
* [[spoiler:BackFromTheDead]]
** [[spoiler:CameBackWrong]]
* BecauseYouWereNiceToMe: She ordered [[spoiler: the infected humans]] to [[YouWillBeSpared avoid hurting Tatsuya]] because he was nice to her.
* BlessedWithSuck: [[spoiler: She has the ability to turn her tail into legs and move around on land, but can't go far from the ocean]]. See HeroicRROD below.
* CryCute / InelegantBlubbering: She does [[ one]] or [[ another]].
* CuteMute
* FishOutOfWater: [[spoiler:Quite literally, since she starts living in Tatsuya's house.]]
* ForcefulKiss: [[spoiler:The way she infects humans on land.]]
** [[spoiler:At the end of chapter 38, she gives one of these to Tatsuya.]]
* GirlinessUpgrade: After becoming a new [[spoiler:PrettyFreeloader]], Nagisa put her intro a cute dress.
** HairDecorations: Two blue hair clips.
* GirlishPigtails: At school.
* GoingCommando / VaporWear: She doesn't seem know what a bra is or what it's for.
* HeroicRROD: [[spoiler:If she stays on land too long, Sheila's body will start to painfully rot, as it's a human corpse from neck down. She can only live safely in the sea]].
* IdenticalStranger: To Tatsuya's mother, Umi. The reasons for this have not yet been revealed.
* ImpossibleHourglassFigure
* InnocentFanserviceGirl
* InSeriesNickname: Shii-chan.
* MoralityPet: To her elder sister.
* NonHumanSidekick: Anna the sea slug was this for her, aside from being her best friend.
* [[spoiler:PrettyFreeloader]]
* [[spoiler:OffWithHerHead: The way Seine killed her 300 years ago]].
* OurMermaidsAreDifferent
* RapunzelHair
* ScarsAreForever
* SchoolUniformsAreTheNewBlack: Sheila is always showed wearing her uniform. [[spoiler:Justified; it seems to be her [[LimitedWardrobe only wardrobe]].]]
* ShrinkingViolet
* SingleWomanSeeksGoodMan: Sheila fell for Tatsuya because he was the only person aside from Ariel to show her kindness.
* SproutingEars: Or Sprouting Dog Tail, in her case.
* TalkingWithSigns: She carries around a notebook wherein she writes what she wants to say. She also learns some HandSignals from Tatsuya.
* ThrowTheDogABone: [[spoiler:Ariel gave her Enati's lower body after she died so now she's a complete mermaid again]].
** [[spoiler:And after Seine give up Aion, this returned to her and give her back her voice]].
* TragicKeepsake: She conserved the necklace that Nagisa chose and Tatsuya brought for her [[spoiler: even after returning eternally to the sea]].
* [[spoiler:UnusualEars: As a mermaid.]]

!!Simon Magus

Simon was Seine's third foster father and the one person who showed her kindness when she was young. He was known as a magician in his time and was also known to be 1700 years old (about 300 years ago in present time of the story). He bore the "Aion" mark, just like Seine does at the present time, but he had it across the right shoulder, and was able to use Aion's power for his own, unknown, reasons. [[spoiler:He died eaten by the mermaids.]]

* BadassGrandpa
* BecauseYouWereNiceToMe: Seine decided go with him because Simon was nicer to her than her current foster father.
* BigOlEyebrows: In comparison to the rest of the cast.
* CoolOldGuy
* CrouchingMoronHiddenBadass: The child raising fanatic was Seine's predecessor at hunting mermaids.
* DissonantSerenity: Since he's [[{{Immortality}} immortal]] and knows [[GoodThingYouCanHeal his wounds can't be fatal]], he's always cheerful even in in the face of danger.
* [[spoiler:EatenAlive]]
* FaceFramedInShadow: Until his formal introduction,
* GrandPaWhatMassiveHotnessYouHave: Let's face it, for a man who look in his 50-something and actually is 1700 years old, Simon look very good.
** Even Shizuki admited she would totally do him if she could had decided stop his aging a little more lately so he would be older enough of her standard.
* ItsNotYouItsMyEnemies: Tried to leave Seine somewhere safe without telling her before he went to fight the mermaids...
** ShooTheDog: ...but Seine went back to him, right in the middle of the fight, and yelling at her to run to safety[[spoiler: distracted him enough that the mermaids were able to attack him and eat him]].
* LikeAGodToMe: Seine quoted this trope word-for-word to describe her admiration and love of Simon.
* ParentalSubstitute: To Seine.
* PosthumousCharacter
* PowerTattoo
* [[Really700YearsOld Really 1700 Years Old]]
* ScarfOfAssKicking
* SpellMyNameWithAnS: Some translations write his name as "Shimon", Shimon being the Japanese pronunciation of Simon.
* [[spoiler:OurGhostsAreDifferent]]

!! The Mermaids
They are Seine's prey, in revenge for Simon's death. In-universe, they are the ones who sank Atlantis, Mu, and Port Royal and destroyed Pompeii in order to collect psyche from humans. Up til now, 5 of the 7 mermaids have been introduced. They are Ariel the eldest, Bella the second, Enati the third, Anshu the fourth, and [[spoiler:Sheila the seventh]].

* AffablyEvil: Ariel is less of a {{Jerkass}} than the rest of her sisters and she and Shizuki only had a PassiveAggressiveKombat when they saw each other.
* AmbiguouslyBrown: Ariel.
* AnimeHair: Bella.
* AntiVillain: Ariel, [[SlidingScaleOfAntiVillains type 1]].
* [[spoiler:ArtificialLimbs:]] Enati.
* AsskickingEqualsAuthority: No matter if you're younger, if you're powerful enough you can take your older sister's place in the ladder. [[spoiler:That precisely is Enati's plan]]
* AttackOfThe50FootWhatever: Ariel. She's big enough to easily pick up her younger sisters bodily in one hand, and her face alone is taller than Seine.
* BadWithTheBone: Enati [[spoiler:uses Simon's skull to make an illusion of him]].
* BerserkButton: [[BigSisterInstinct Hurting or bullying her youngest sister]] will set Ariel off.
* {{Big Bad}}s
* [[BigBrotherBully Big Sisters Bullies]]: All but Ariel pick on their youngest sister.
* BigSisterWorship: Anshu to Enati.
** Later to [[spoiler:Sheila]] after she get her lower body.
** Later discovered that the fifth and the sixth have this towards Bella.
* BreakThemByTalking: Enati and Anshu to [[spoiler:Sheila]].
* TheBrute: Bella. She's sadistic, ruthless, and loves to cause pain.
* CollectorOfTheStrange: Anshu likes to collect [[spoiler:human parts]].
* {{Cute Monster Girl}}s: All of them (even Ariel), except for Bella.
* CryCute: Ariel, when she cries, cries from the bottom of her heart and looks so adorable.
* TheDreaded: Bella is feared by all her younger sisters. Ariel, being the eldest and strongest one, is this to Bella.
* TheDitz / TheDarkChick: Anshu.
* EvilGenius: Enati.
* FangsAreEvil
* FemmeFatalons
* GiantWallOfWateryDoom: Their ultimate weapon.
* HellishPupils
* [[spoiler:ImAHumanitarian]]
* [[IHaveNoSon I Have No Sister]]: Bella to [[spoiler:Sheila]].
* JustBetweenYouAndMe: The fight between Seine and Ariel.
* LotusEaterMachine: [[spoiler:Ariel put Seine in one of this]].
* [[spoiler:MasterOfIllusion]]: Enati.
* MindControl: Via Mushis.
* MulticoloredHair: Anshu.
* [[spoiler:OffWithHerHead]]: Enati.
* OurMermaidsAreDifferent
* RapunzelHair: Ariel, Bella and Anshu.
* ShortHairWithTail: The sixth mermaid.
* SiblingRivalry