Voiced By: Kaito Ishikawa
An isolated and calm young man who became interested in cases of "Adolescence Syndrome" after his sister started getting injuries and cuts on her body as a result of cyberbullying. He has a habit of being blunt and doesn't seem to care if others look down on him for being outspoken or disruptive.
- Big Brother Instinct: Is very protective of Kaede's well-being, even going to the extent to throwing away his cell phone for her sake.
- Brutal Honesty: Sakuta is very blunt and rarely keeps things to himself.
- Chick Magnet: Halfway through the series, Sakuta's attracted a fair amount of female companions, although they never border on being a traditional harem.
- Chivalrous Pervert: Sakuta shamelessly admits he finds Mai sexually attractive several times, even when she is right in front of him, but he respects her and never tries to do anything. In general, pretty much any girl who isn't his sister can become the target of Sakuta's lewd attention if they annoy him, but he never goes beyond harassing them with words.
- Combat Pragmatist: Resorts to this when he ends up fighting Tomoe's suitor from the basketball club (and thus is much physically stronger than Sakuta), first feigning a punch and surprising him with a low kick instead, then kicking him in the face while he's stunned.
- The Comically Serious: Sakuta has a very deadpan manner about him, even when saying inappropriate things about his female classmates. This is mostly his way of joking around, however.
- Deadpan Snarker: Makes sarcastic remarks from time to time, making him engaging in Snark-to-Snark Combat with Mai entertaining.
- Mistaken for Pedophile: Tomoe and Sakuta's first meeting occurs when she mistakes him for a lolicon for trying to help a lost girl. They later agree to literally kick each others' asses in broad daylight.
- Noodle Incident: Sakuta is allegedly responsible for hospitalizing three people. He claims this is untrue but doesn't bother fighting the rumors at school.
- O.O.C. Is Serious Business: The guy emotes very little beyond smiles and mild surprise, even at his happiest, which makes it all the more shocking when the events of the final episodes leave him a screaming, crying mess.
- Scars Are Forever: He has a giant, claw-like scar on his chest. The wound appeared out of nowhere during Kaede's original case of the Syndrome and landed him in the hospital. Doctors couldn't figure it out beyond probable self-harm, starting the rumors about "the hospitalization incident". For something he got two years ago, it's still very pronounced. Given that the wound violently reopens when Kaede's original personality comes back, there's a strong implication that it is its own case of Adolescence Syndrome revolving around loathing his inability to help his little sister.
- Single-Target Sexuality: Despite meeting a new girl every month with a new Adolescence Syndrome, Sakuta has only expressed interest in Mai. This is downplayed when he reveals that he did have feelings for high school Shoko, but moved on.
- Weirdness Magnet: Somehow he keeps winding up involved in cases of Adolescence Syndrome.
Voiced By: Asami Seto
An attractive third-year who attends the same high school as Sakuta and a notable child actress. By the start of the series she's on an indefinite hiatus from her work and despite her celebrity status, doesn't try to stand out at school much.
- Animal Motifs: Mai occasionally wears a Playboy Bunny costume and has a rabbit hair tie. She also wears a rabbit ear headband, and her phone has a rabbit case.
- Cool Big Sis: To both Kaede and Nodoka.
- Deadpan Snarker: Her and Sakuta's tendency to engage in Snark-to-Snark Combat make them a very entertaining pair to watch.
- Everyone Has Standards: She made it clear that she was not willing to do any commercials that put her in swimsuit type clothing. When her mom did so anyway, she felt betrayed and quit immediately afterward. She felt extremely uncomfortable about showing off her body when she was just a middle schooler, yet her mother went over her head and forced her to do it for the money anyway.
- Former Child Star: A very grounded example, she misses acting in dramas, but distances herself from her work because she felt her mother was only using her for money.
- Iconic Outfit: The playboy bunny costume she's seen wearing in the first episode, and only once afterwards.
- I Just Want to Be Normal: Mai takes a hiatus from acting for this reason. Unfortunately, this triggers her Adolescence Syndrome, causing her to be invisible to an unspecified amount of people that seemingly keeps rising. Sakuta and at least some other students at school still see her just fine though, until even that starts to fade.
- Obfuscating Insanity: At first glance, you'd think Mai was wearing a Playboy Bunny suit for fanservice but it's revealed that she wears it so she can recognize when she is and isn't invisible.
- Playboy Bunny: Mai dresses like this in order to identify areas where she is invisible.
- Rescue Romance: Sakuta saves Mai from becoming an Unperson by shouting out his feelings for her. Mai accepts Sakuta's feelings and start dating each other.
- Sustained Misunderstanding: Notably averts it both times it would've happened in any other story, be it hearing Sakuta out and accepting his explanation when she sees Tomoe on top of him, and then, a few episodes later, holding a press conference confirming her relationship with him when a paparazzi photo of them together makes the rounds.
- The Tease: Is coquettish towards Sakuta when they are dating, although it is clear that Mai loves him.
- Unperson: Mai worries she's becoming this until only Sakuta will be able to see her, or worse. Heck, even her name becomes blurry when seen written by those who have forgotten her.
Voiced By: Nao Touyama
A first-year girl with a strong desire to fit in with her classmates.
- A-Cup Angst: Rather resentful of how the food she eats tends to go to her stomach and hips instead of her chest.
- Becoming the Mask: Sakuta agrees to her request of pretending to be her boyfriend at least till the end of the school year but she ends up developing feelings for him. Both Sakuta and Tomoe realize this, as time keeps repeating itself because Tomoe has not come to terms with her own feelings. It's only when Sakuta confronts her about it that she lets Sakuta go.
- "Groundhog Day" Loop: When affected by the Adolescent Syndrome, Koga is somewhat capable of doing this. Sakuta is the only other person who remains aware. Futaba dubs Koga as the Laplace Demon. It is like Koga keeps rolling the dice several times to get the future she desires. This eventually transforms into The Story That Never Was and Koga is cured of the Adolescent Syndrome.
- Hartman Hips: According to Sakuta, Tomoe has this type of hips. She's none too pleased when he points that out. For example, when they both went to the beach and had a sand castle competition, Tomoe's castle survived the tide because her butt had left massive indentations in the sand that stopped the water.
- Hopeless Suitor: Even though in the beginning Sakuta had made it clear to Koga that he loves only Mai, Koga cannot help herself fall in love with Sakuta. This triggers another "Groundhog Day" Loop. Sakuta forces Koga to be honest with herself and that is the key to escape the loop.
- The Idiot from Osaka: A bit of a touchy subject for her. She moved to Tokyo in high school and reinvented herself upon arrival into a 'fashionable' girl that could thrive in a big city.
- I Just Want to Be Special: Tomoe serves as Mai's inverse. Once a self-proclaimed bumpkin, her move from the countryside to the city saw her change how she looked and acted so that she could be popular and have friends. However, this is portrayed in a more negative light, as she pushes herself to stay involved no matter what because of her fear that even a day away from them could cost her social life. That desire to maintain those relationships and status is what triggers her Adolescence Syndrome, which Sakuta is well-equipped to help her with due to his inability to care about how the world thinks of him.
- I Just Want to Have Friends: This crops up as her main motivation to keep up a 'perfect' image of herself in front of her peers.
- Ooh, Me Accent's Slipping: When she gets flustered or excited.
- Phoneaholic Teenager: She is inseparable from her smartphone and will be texting even mid conversation.
Voiced By: Atsumi Tanezaki
One of Sakuta's only longtime friends and the sole member of the Science Club.
- Brutal Honesty: Rio is quite blunt when addressing Sakuta, usually calling him a rascal.
- Cloning Blues: Rio's Adolescence Syndrome manifests in the form of a second version of herself, identifiable by her hair being in a ponytail and using contacts instead of glasses. Barring looks, both act enough like the Rio that Sakuta has been friends with to the point where he cannot call either of them as a fake.
- Interactions with both versions of Rio piece together that the separation was likely caused by contradictory internal desires, likely about the attention she receives and how she actually goes about it. This self-hatred within her is sort of pacified when Sakuta reaffirms her friendships with him and Yuuma and allowing her to at least bear with it, causing both Rios to merge. Interestingly enough, the supposed "real" Rio whose appearance closely mirrored her original look is the one that ends up disappearing when the merge happens.
- Cry Cute: The normally stoic Futaba cries when realizing how much her friends care about her, Sakuta even comments how her crying is cute.
- D-Cup Distress: Rio has some body issues because she started developing early, and was harassed because of it. The sexy selfies she took were done as a coping mechanism.
- Deadpan Snarker: Her sarcasm is actually sharper than Sakuta's.
- Emotionless Girl: She never emotes much, leaving a rather cold impression as a result. We see more of her inner feelings and what makes her tick halfway through the season.
- Literal Split Personality: Her Adolescence Syndrome separates her into two people. Unusually, the two halves differ very little from each other.
- The Not-Love Interest: Breaking the trend from Mai and Tomoe, Rio's case of Adolescence Syndrome is initiated by unrequited feelings not for Sakuta, but their mutual friend Yuuma. Rio's relationship with Sakuta is strictly in the Vitriolic Best Buds territory, but she later admits that she's just as afraid of their friendship vanishing as well now that he has a girlfriend.
- The Smart Girl: She constantly provides Sakuta with information in order to solve a case of Adolescence Syndrome.
- The Stoic: Rarely shows any emotion, and speaks at nearly all times with a calm, quiet, almost monotone voice. The only time she ever raises her tone is when Sakuta comments on her chest size, which is a touchy subject for her.
- Tomboy Ponytail: When Rio's Adolescence Syndrome kicks in and a second version of herself is split off, she has one of these by default while switching out her glasses with contacts. Upon reuniting both halves, she occasionally keeps the ponytail but returns to wearing glasses instead.
Voiced By: Yurika Kubo
Sakuta's homebound little sister who hasn't attended school since her case of Adolescence Syndrome resolved. Often seen sitting around the apartment in her panda pajamas.
- Amnesiac Dissonance: It's even treated as full-blown dissociative identity disorder. Kaede post-amnesia has a different personality, tastes, habits, speech patterns, dominant hand, everything. She doesn't even consider her old self as the same person, calling her past by a polite "Kaede-san" like a stranger she's never met.
- Animal Motifs: Kaede seems to be closely tied to pandas. Around the apartment, she wears a panda onesie.
- Clingy Jealous Girl: Played for Laughs with Kaede, who always puts on a defensive front any time she sees Sakuta bringing home a new girl, and also calls him a gigolo.
- Death of Personality: At the end of episode 12, Kaede recovers her childhood memories while losing the last two years, essentially ending the Kaede the audience has known.
- Hikikomori: Ever since middle school it seems, she's painfully shy and can barely talk to strangers.
- Keigo: Always speaks in polite forms, even with her own brother. Most likely a result of her trauma.
- Laser-Guided Amnesia: Possibly part of her Adolescence Syndrome, Kaede can't remember anything of her life at school.
- Little Sister Heroine: Gives a few teasing nods to being this, but Sakuta doesn't indulge her. It's revealed that she deliberately fashioned herself to be the best little sister ever as thanks for Sakuta acknowledging her as her own person rather than a broken version of the old Kaede, and might have gone a bit overboard on the affective side.
- Hikikomori: Kaede has dropped out of school, never leaves home and even avoids technology. She decides to go back to school at the end of episode 13.
- Nice Girl: Prior to her history with her cyberbullying from her classmates, she is described as having a kind personality. Even after shutting herself from the outside world, she maintains some of her polite mannerisms.
- Shrinking Violet: Thanks to her history with bullying, Kaede becomes very meek around strangers, and her voice barely rises above a whisper.
- Tender Tears: Does this after finally stepping out of the apartment.
- Third-Person Person: Always refers to herself by her own name. This is how she copes with her identity after getting Adolescence Syndrome-induced amnesia. Once she regains her memories, she drops this habit.
- Trauma Button: It's mild, but after her troubles with cyberbullying, she visibly gets triggered when phones ring around her. It takes a lot of work and cheering for her to even answer the landline for the first time.
Voiced By: Inori Minase
A mysterious girl from Sakuta's past who comforted him during his sister's case of Adolescence Syndrome. Also the name of a middle-schooler who adopted a stray cat that stays with Sakuta and Kaede.
- Imaginary Friend: This theory is advanced by Futaba for the older version. There are strong arguments either way.
- Mysterious Protector: The older Shoko only seems to appear to Sakuta when he's at his absolute lowest, does her best to cheer him up, and then disappears again when he feels better.
- One Steve Limit: Shoko seemingly disappears from the high school Sakuta later attends. Later, he meets a middle schooler who has the same name and bears a striking resemblance to her.
Voiced By: Maaya Uchida
Mai's half-sister, and part of the idol group Sweet Bullet.
- Always Someone Better: She views her sister Mai as this, especially as her mother often complains about how she isn't measuring up to Mai at all. It's driven home particularly fiercely when she sees Mai performing on stage as her and doing far better on her first try, even covering for the main singer's mistakes and getting praised by Nodoka's mother afterward.
- Aw, Look! They Really Do Love Each Other: What ultimately helps her switch back is learning that Mai kept a secret box full of all her correspondence to her sister.
- "Freaky Friday" Flip: She switches places with Mai as part of her Adolescence Syndrome, in part due to her desire to be her.
- Hard Work Hardly Works: Played with. Her feelings of inferiority seem to be based at least in part in the idea of 'no matter how hard I work, my sister does better effortlessly.' After seeing Mai's work schedule and performances, however, she realizes that it's the opposite: Mai works ridiculously hard at her acting career and Nodoka just doesn't have the same work commitment and probably never will.
- Implausible Hair Color: Subverted. Flashbacks show her with brown hair and eyes, meaning it's probably dye and contacts worn for the sake of standing out more.
- Suicide by Sea: She runs into the sea intending to drown herself, supposedly. Sakuta calls her out on it being a dick move because she obviously wanted him to stop her.
- "Well Done, Son!" Guy: The fundamental difference between her and sister is implied to be that Mai is an actress because she loves the job, Nodoka just wants her mom's approval and thus can't muster up the same passion. Fortunately for her, it's implied that unlike Mai's mother, Nodoka's really does care about her and worries whether she's doing the right thing.