The main male protagonist. Since he tasted Urabe's drool, the two have entered into a strange love relationship.
- Accidental Pervert: Rare for a manga/anime romance protagonist, he only gets a couple of these incidents on his record. Akira also outright averts this when he trips and is about to fall onto Urabe in her apartment, where he deliberately causes himself to fall into her bedpost instead.
- Chivalrous Pervert: In spite of his teenage libido rearing its head when Urabe is around, Akira never lets it override his morals or cause him to hurt his girlfriend. His resolve does waver when especially erotic situations arise, but that's to be expected at his age.
- Nice Guy: Akira is a genuinely nice guy who, while sometimes confused about Urabe's mysterious ways, does his level best to treat her right.
- Ridiculously Average Guy: He doesn't excel at anything in particular, being a pretty average Japanese teenager in all regards.
The main female protagonist. A really weird girl who enjoys cutting things up with her "panty scissors".
- Adjective Noun Fred: Urabe's the eponymous Mysterious Girlfriend X for which the series is named after.
- Aggressive Submissive: While she prefers to maintain control of how Akira can touch her, the one time she gets caught off guard sees her become suddenly submissive, which only stopped because he got ahold of himself.
- Beautiful All Along: She really starts turning heads in school just by getting a different haircut that lets her face be visible, getting this reaction from the boys in class. Akira gets rather jealous when boys start talking about how beautiful she is.
- Clingy Jealous Girl: Urabe, in turn, gets rather jealous when Akira becomes a fan of the idol Momoka Imai and shows her the fanbook he bought. She calmly asks him what is his favorite picture is, waits for him to show it to her, then Urabe slices the entire book to pieces... even though Akira only likes the idol since she reminds him of Urabe.
- Cloud Cuckoo Lander: Unsurprisingly, Urabe is a rather bizarre girl, albeit a more focused example than most. She clearly thinks in a far different way than most around her, and habitually keeps a pair of scissors hidden on her underwear as a weapon. Putting aside the whole "psychic powers via saliva" thing, she gets this reputation on the first day of school by having a laughing fit in the middle of class.
- Covert Pervert: She has a lot of self control, but she admits that she's just as perverted as Akira. Part of this is helped along by how she refuses to let Akira notice when she gets affected by his saliva or when he shows up in her dreams.
- Defrosting Ice Queen: At the beginning, Urabe is cold and aloof to the boy that she knows she'll lose her virginity to, but she warms up to him as they establish a bond of love and trust.
- Double Standard: Abuse, Female on Male
- Hates Being Touched: A major Berserk Button for Urabe is Akira attempting to hug her without Urabe's prior permission, at which point something of his will be getting sliced to ribbons.
- Heterosexual Life-Partners: With Oka, in spite of Oka also being addicted to Urabe's saliva.
- Power of Trust: They even have no problems giving their boyfriends erotic picture with both of them in bikinis. Although Urabe was hesitant at first.
- Hidden Buxom: Although less evident through her clothes, Urabe has pretty big breasts for her age.
- Ms. Fanservice: Together with Oka, they are the biggest suppliers of fanservice in the series.
- Peek-a-Bangs: Urabe's signature hairstyle covers her eyes most of the time, with her eyes only being visible when she's making a point or being serious.
- Polar Opposite Twins: Unrelated example with Momoka, seeing as they look like each other. Momoka is well-known for being outgoing and smiling frequently, whereas Urabe is reserved and stoic.
- Property of Love: Urabe labels herself as Tsubaki's 'property' in one chapter/episode.
- Shear Menace: With a pair of hand scissors that she habitually carries, that she can Blade Spam with high precision.
- The Stoic: In comparison to Akira's excitable nature, Urabe is pretty unflappable and calm most of the time...
- Not So Stoic: Except when her boyfriend says something that really affects her or when his saliva has powerful emotions in it, in which case her facade cracks and shows that she's just as prone to embarrassment as he is... or when Oka tries to look up her skirt, or that time when Oka revealed she took a photo of Urabe when she was naked.
- Sugar-and-Ice Personality: Mostly ice in public and early on in her relationship with Akira, with the sugar coming up when Akira shows his love for her.
- Tsurime Eyes
- When She Smiles: Her smiles are few and far between, but she usually blows Akira away whenever she does.
Urabe's first friend.
- Beta Couple: With Ueno, as they have a somewhat normal relationship in comparison to Akira and Urabe's weird one.
- Cloud Cuckoo Lander : She's only slightly less of a mystery then Urabe, even without the drool powers.
- A Friend in Need : Hangs around Urabe even when she wasn't "officially" her friend.
- Heterosexual Life-Partners: With Urabe.
- Power of Trust: They even have no problems giving their boyfriends erotic picture with both of them in bikinis.
- Ms. Fanservice: Together with Urabe, Oka's one of the biggest suppliers of fanservice.
Tsubaki's friend and Oka's boyfriend.
- Beta Couple: With Oka.
Tsubaki's elder sister.
- Promotion to Parent: She considers it her duty to raise her brother properly in place of their deceased mother.
Tsubaki's former classmate and first crush in middle school.
- Manipulative Bitch: After her own failings in love, she tries to steal Tsubaki for herself.
- Psycho Ex-Girlfriend: Psycho Ex-Crush.
Popular idol who is nearly identical to Urabe.
- A-Cup Angst: The biggest physical difference between her and Urabe.
- I Just Want to Be Normal: Downplayed, she enjoys her life as an idol but feels lonely and wants to find love.
- Identical Stranger: With Urabe to the point that you can consider them as "twins".
- Idol Singer
- Kick Chick: Has habit of roundhousing people, as Tsubaki can attest to.
- Mistaken Identity: As Urabe by Tsubaki.
- Polar Opposite Twins: Unrelated example with Urabe.
- Twin Switch: With Urabe during her concert and later in the promotion of Urabe's school movie.
Tsubaki and Urabe's beautiful classmate.
- Hopeless Suitor: She develops feelings for Tsubaki.
- Love Confession: She confesses to Tsubaki that she would not mind to be "second one" for him if he wants.
- Secret Relationship: She offers this to Tsubaki as an option in front of Urabe.
- Tareme Eyes