The characters of Hekikai no AiON include:
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.
- Abusive Parents: Her second father abused and traumatised her to the the point she is unable to swim.
- Animal Motifs: Cats. So much that she portrayed as a Cat Girl in an omake.
- Berserk Button: Shizuki and Yuzuki. Also anything mermaid-related.
- Broken Bird: The first 4 years of her life were horrible, her Parental Substitute 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.
- Everyone Calls Him "Barkeep": Seine is frequently called "witch" instead of her real name.
- Expy: Of Karin, though only in appearance.
- Girl in a Box: Lived in a cardboard box prior to meeting Tatsuya.
- Huge Guy, Tiny Girl: With 19-year-old Tatsuya.
- I Gave My Word: When she finds someone who asks her to stay by his/her side, she stays until they die, tell her to leave, or obstruct her revenge, because she used to be completely alone, too.
- Important Haircut: She used to have a mild example of Rapunzel Hair, but she cut it when she ultimately decided to follow the You Killed My Father path.
- Innocent Fanservice Girl: More like "Indifferent Fanservice Girl".
- Le Parkour: With Aion's help. Tatsuya even compares her to a roof-jumping cat.
- Neet: After the Time Skip.
- Not Growing Up Sucks: 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.
- Parental Abandonment: She doesn't know anything about her biological parents.
- Sugar-and-Ice Personality: Later, she evolved to a Tsundere towards Tatsuya.
- Super Drowning Skills: 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.
- Teens Are Short: Her height is 1.46m (4' 7").
- The Nose Knows: She claims infected humans have a different smell than normal humans, and she's learned how to distinguish it over time.
- When She Smiles: Just look at her. Tatsuya is out of breath.
- You Gotta Have Blue Hair: She has green hair.
- You Killed My Father: Her Revenge motivation.
- Zettai Ryouiki: Mostly Grade B, sometimes Grade A.
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.
Four years later, he starts going to University at the city.
- Because You Were Nice to Me: After Seine encouraged him by saying he can become a "high caliber" man, he fell for her and wanted to be acknowledged by her.
- The Big Damn Kiss: 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.
- Break the Cutie: 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.
- Dragged into Drag: Forced to dress up as a meido as part of his class's cafe staff. And the girls rejoiced, but no one can deny he looks so cute as a female.
- Heroic BSoD: At his parents' funeral. Understandable.
- Heroic Willpower: Why he didn't kill Seine even after being infected.
- I Got Bigger: Though 19-years-old Tatsuya looks quite the same as 16-years-old Tat-chan, just bigger.
- In Touch with His Feminine Side: 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.
- Kid with the Leash: He's the only one able to control and stop Seine when she's on a rampage. Also, he could be this to Sheila too.
- Lethal Chef: As Shizuki discovered. Only Seine is not bothered by his cooking.
- Living Emotional Crutch: He became more or less this to Seine after his Journey to the Center of the Mind 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.
- Lonely Rich Kid: Until Seine promises him to stay.
- Non-Action Guy: Though he's helpful in other ways.
- Oblivious to Love: 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.
- Parental Abandonment: His parents died a week before the start of the story.
- Say My Name: "MIYAZAKI-SAN!"
- Secret Keeper: He promises to keep the secret that Sheila is actually a mermaid after doing a Pinky Swear with her.
- Single-Target Sexuality: Seine.
- Teens Are Short: 1.62m (5' 4") at the age of 16.
- Together in Death: With Seine.
- You Are Not Alone: To Seine.
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.
- Belligerent Sexual Tension: With Minato.
- Berserker Tears: Cried these while choking Seine to death.
- Cannot Spit It Out: She just can't say: "Tatsuya, I love you."
- Control Freak: Mild example. Since she is in charge or her house, she's very strict even with her father.
- Damsel in Distress: When she was infected by Mushis.
- Expy: Of Maki from Karin. They have a similar appearance to each other and are childhood friends to the main character. Nagisa's Belligerent Sexual Tension with Minato also parallels to Maki and Yuuji's Slap-Slap-Kiss relationship.
- Expository Hairstyle Change: After the Time Skip, she's seen with a bob instead of her long side ponytail.
- Fanservice with a Smile: She works as a waitress in a cafe.
- First-Name Basis: With Minato. She also used to have this with Tatsuya, but since everybody thought she was 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 the Mushi incident.
- Heroic Self-Deprecation: She thinks she's a bad, selfish person, unworthy of Messianic Archetype Tatsuya. She gets better.
- More Than Mind Control: 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.
- Plot Allergy: 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 her infection.
- Single Woman Seeks Good Man: She fell for Tatsuya when he defended her from accusations of being a gold digger.
- Unlucky Childhood Friend
- You Gotta Have Blue Hair: She has purple hair.
Tatsuya's friend and Nagisa's neighbor. More worldly and savvy than his friend, he notices that Seine is not a normal girl.
- Belligerent Sexual Tension: With Nagisa.
- The Confidant: He's the only who Nagisa can talk with about her crush on Tatsuya and her allergy to dog hair.
- Sensitive Guy and Manly Man: Manly man to Tatsuya's sensitive guy.
- Tall, Dark, and Handsome: With some degree of Tall, Dark, and Snarky
- Those Two Guys: With Nagisa.
Tatsuya's little second cousin.
- Bound and Gagged: Her own parents did this to her when they were Brainwashed and Crazy.
- Break the Cutie: Chapter 3 and 4 did this to her, but thanks to Laser-Guided Amnesia as Reset Button, it was like this never happened.
- Canine Companion: After Croquett helped her, she started to have a soft spot for him.
- Children Are Innocent:...but she can tell that Tatsuya's house is full of perverts.
- Cool Big Sis: She sees Seine as this after Time Skip.
- Hair Color Dissonance: Is she a brunette, dark blonde, or even a strawberry blonde?
- Hilariously Abusive Childhood: Her father wants to pay off his debts by marrying her to Tatsuya and getting at Tatsuya's inheritance; she declares her parents barely pay attention to her and just worry about money, and when they try to care, they just shame her. All Played for Laughs.
- Ms. Exposition: She explained the formation of fog, which was a plot point in chapter 31.
- Out of Focus: Lampshaded in chapter 25. This may change now that she lives in Tatsuya's house.
- Spoiled Brat: She grows out of it after the Mushi incident with her parents.
- Token Mini-Moe: And first victim of Yuzuki's Forceful Kiss...at least this time.
- Wise Beyond Her Years: She's the most reliable one in Tatsuya's house.Minagi: With that kind of parents around you, you'll grow up to be reliable even if you don't want to.
- Yaoi Fangirl: She may be joking, though.
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.
- Closer to Earth: He says to Tatsuya that he can't go around picking girls to live in his house. And he's right.
- Like Brother and Sister: Non-romantic way. He has no blood tie with Tatsuya or Minagi, but he's a member of the family.
- Loving a Shadow: He seems to have a crush on Sheila because she looks like a carbon copy of Umi.
- No Indoor Voice: Most of his dialogue is yelling at Seine or Tatsuya, usually while crying.
- Older Than He Looks: He looks like in his early 20, but it's 27. After the Time Skip, he looks practically the same but now he's 31.
- Scary Shiny Glasses: When pissed off.
- Team Chef: After Time Skip. He became this after moving to Tatsuya's house since Yuzuki was Put on a Bus 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 Lethal Chef). 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).
- Animal Eye Spy: Shizuki can see through the eyes of Chibi Piyo-piyo. and since Croquet eat Chibi Piyo-piyo, she can see what Croquette sees.
- But Now I Must Go: 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.
- Ill Girl: Her body is so weak that she can't swim in pools because of the chlorine, gets tired easily, and Tatsuya's food almost killed her. Luckily she's immortal.
- Meaningful Name: Shizuki means "calm moon". Also, her former name. "Pistis Sophia" is the female divinity of gnosticism that means "Faithful Wisdom".
- Multicolored Hair: Black, with blonde bangs.
- Neet: She does absolutely nothing but eat and sleep. Partially justified as she's an Ill Girl.
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 Comedic Lolicon (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.
- Apologetic Attacker: When "attacking" Seine at her sister's demand.
- Big Damn Heroes: She saves an exhausted Seine from Bella's attack.
- Boobs of Steel: The physically strongest character in the series and one of the bustiest.
- But Now I Must Go: 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.
- The Ditz: Shizuki even said that Yuzuki has the mind of a kindergartener.
- I'm Taking Her Home with Me!: Yuzuki seems to have this sort of reaction to every small, cute child she sees.
- Multicolored Hair: Black with blonde bangs.
- No Sense of Direction: She gets lost inside a school, mostly because of her ditziness.
- Theme Twin Naming: With Shizuki.
She was introduced as a new transfer student, and Tatsuya is given the assignment of teaching her sign language. 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.
Four years later she meets Tatsuya as he convinces her to stop thinking of revenge but 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.
- Because You Were Nice to Me: She ordered the infected humans to avoid hurting Tatsuya because he was nice to her.
- Blessed with Suck: She has the ability to turn her tail into legs and move around on land, but can't go far from the ocean.
- Fish out of Water: Quite literally, since she starts living in Tatsuya's house.
- Forceful Kiss: The way she infects humans on land, At the end of chapter 38, she gives one of these to Tatsuya.
- Girliness Upgrade: After becoming a new Pretty Freeloader, Nagisa put her intro a cute dress.
- Going Commando: She doesn't seem know what a bra is or what it's for.
- Heroic RRoD: 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.
- Identical Stranger: To Tatsuya's mother, Umi. The reasons for this have not yet been revealed.
- Non-Human Sidekick: Anna the sea slug was this for her, aside from being her best friend.
- Off with Her Head!: The way Seine killed her 300 years ago.
- School Uniforms Are the New Black: Sheila is always showed wearing her uniform. Justified; it seems to be her only wardrobe.
- Single Woman Seeks Good Man: Sheila fell for Tatsuya because he was the only person aside from Ariel to show her kindness.
- Sprouting Ears: Or Sprouting Dog Tail, in her case.
- Talking with Signs: She carries around a notebook wherein she writes what she wants to say. She also learns some Hand Signals from Tatsuya.
- Throw the Dog a Bone:
- Ariel gave her Enati's lower body after she died so now she's a complete mermaid again.
- After Seine give up Aion, this returned to her and give her back her voice.
- Tragic Keepsake: She conserved the necklace that Nagisa chose and Tatsuya brought for her even after returning eternally to the sea.
- Unusual Ears: As a mermaid.
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. He died eaten by the mermaids.
- Because You Were Nice to Me: Seine decided go with him because Simon was nicer to her than her current foster father.
- Big Ol' Eyebrows: In comparison to the rest of the cast.
- Crouching Moron, Hidden Badass: The child raising fanatic was Seine's predecessor at hunting mermaids.
- Dissonant Serenity: Since he's immortal and knows his wounds can't be fatal, he's always cheerful even in in the face of danger.
- Face Framed in Shadow: Until his formal introduction,
- It's Not You, It's My Enemies: Tried to leave Seine somewhere safe without telling her before he went to fight the mermaids... but Seine went back to him, right in the middle of the fight, and yelling at her to run to safety distracted him enough that the mermaids were able to attack him and eat him.
- Like a God to Me: Seine quoted this trope word-for-word to describe her admiration and love of Simon.
- Parental Substitute: To Seine.
- Silver Fox: 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.
- Spell My Name with an "S": Some translations write his name as "Shimon", Shimon being the Japanese pronunciation of Simon.
The MermaidsThey 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 Sheila the seventh.
- Affably Evil: Ariel is less of a Jerkass than the rest of her sisters and she and Shizuki only had a Passive-Aggressive Kombat when they saw each other.
- Asskicking Equals Authority: No matter if you're younger, if you're powerful enough you can take your older sister's place in the ladder. That precisely is Enati's plan.
- Attack of the 50-Foot Whatever: Ariel. She's big enough to easily pick up her younger sisters bodily in one hand, and her face alone is taller than Seine.
- Big Sister Bully: All but Ariel pick on their youngest sister.
- Big Sister Worship:
- Anshu to Enati, later to Sheila after she get her lower body.
- Later discovered that the fifth and the sixth have this towards Bella.
- Break Them by Talking: Enati and Anshu to Sheila.
- The Brute: Bella. She's sadistic, ruthless, and loves to cause pain.
- Collector of the Strange: Anshu likes to collect human parts.
- Cry Cute: Ariel, when she cries, cries from the bottom of her heart and looks so adorable.
- The Dreaded: Bella is feared by all her younger sisters. Ariel, being the eldest and strongest one, is this to Bella.