Japanese manga series
My Monster Secret, known in Japan as , is a Japanese manga series written and illustrated by Eiji Masuda. It was serialized in Akita Shoten's "shōnen" manga magazine "Weekly Shōnen Champion" from January 31, 2013, to February 16, 2017. Akita Shoten collected the series in 22 "tankōbon" volumes.
An anime television series adaptation ran from July to September 2015. AnimeLab has secured streaming rights in Australia and New Zealand.
Seven Seas Entertainment acquired the rights to translate and distribute the manga for North America, with the first volume being released in January 2016.
Plot.
Asahi Kuromine, a normal high school student who supposedly cannot keep a secret, finds his demeanor quickly challenged when he spots his classmate and crush, Youko Shiragami, unfurling a large pair of wings from her back when he tries to propose. He learns Youko is a vampire, and she is only able to attend a normal school on one condition: no one must discover her true identity. This secret becomes difficult to maintain, as Youko herself is an airhead and Asahi's childhood friend Mikan keeps bullying the two of them. Love is blooming between Asahi and Youko but things are not very easy. Later on, Asahi discovers more girls in his school with secrets of their own: a tiny alien who rides a human-sized exoskeleton of herself, a werewolf capable of switching gender at any depiction of a moon, a thousand-year-old demon who secretly runs the school, and more who not only pose a challenge as Asahi tries to maintain Youko's secret, but also secretly like Asahi and despise the idea of Asahi and Youko being a couple, thus trying to separate them.
Voiced by: Natsuki Hanae
Asahi is a second year high school student who seemingly cannot keep a secret, so much his nickname is "leaky basket." He discovers his current crush, Youko, is a vampire when he finds her unfurling her wings after school.[ch. 1] As a result, he is forced to keep her secret, lest she have to drop out of both school and his life. He comes to learn secrets about his other classmates and finds the people around him are not who they seem to be. Often forced to play the straight man in increasingly absurd situations, Asahi often acts as the narrator through his inner monologues. He is also susceptible to projectile nosebleeds when confronted with perverse situations, earning the nickname "Eromine" from Youko. Asahi has a younger sister named Mei, though she is barely mentioned in the story until much later on. As the series progresses, Asahi and Youko begin dating, eventually becoming a couple.
Voiced by: Yū Serizawa
Youko is a classmate of Asahi and the daughter of a human and a vampire. Though she has long canine teeth, many other typical vampiric characteristics are downplayed, such as only getting tanned in sunlight or feeling guilty when entering a room uninvited. She has a Kansai accent, but speaks in honorifics so as not to expose her teeth. While initially coming across as haughty, her true personality is bubbly and absent minded. Her dream is to become a "cool beauty", but has yet to accomplish this goal. Though initially friendly with Asahi, she develops romantic feelings for him, but keeps them to herself because she believes Asahi likes Aizawa. She is also often hungry, due to not being able to eat lunch for fear of revealing her teeth. As the series progresses, Asahi and Youko begin dating, and accidentally becomes pregnant with Asahi's child after regularly sucking his blood, unaware a vampire sucking a person's blood has the same effect as sexual intercourse.
Voiced by: Inori Minase
Nagisa is a classmate of Asahi and Youko, and is also a small alien riding a human-sized robotic exoskeleton of herself. Initially the love interest of Asahi before the series began, she turned him down before he could confess, believing her secret would be discovered if she started a relationship with him. She, like most of Asahi's friends, is aware of his feelings for Youko, and develops feelings for him after the fact. Her alien species is militaristic, and she often uses military terminology when describing things. Her exoskeleton runs on an internal battery must be constantly recharged with an AC outlet and has on many occasions run out of power for comedic effect. She is usually meticulous and serious with her actions, though loses her composure when dealing with embarrassing situations. Nagisa has an older brother, Ryo, whom she initially respects and draws inspiration from, before learning he is less reliable than she initially realized. As a small alien, she is always wearing the same clothes her robot is wearing. Her exoskeleton has a metal screw cockpit opening with an antenna sticking out of it, but no one guesses what she really is despite this. When Youko and Asahi start to date, she decides to put her feelings aside to help them, and remain friends. 
Voiced by: Reina Ueda
Mikan is a childhood friend of Asahi who bullies him relentlessly. She is the president of the Newspaper Club, and has used her position to turn it into a supermarket tabloid. Most of the time, though, the newspaper club members do all the work for her. She secretly likes Asahi, but perceives her lack of femininity as a barrier she cannot overcome, and prefers instead to bully and tease him. She wears fake glasses Asahi won at a carnival when they were younger, and they later become the host of a fortune god in training, though this god's attempts to help only end up causing her more misfortune. Mikan has two younger brothers who occasionally appear in the story. She eventually reveals her feelings to Asahi, who turns her down but also reveals he has always seen her as a girl, and she just never noticed it, much to her own regret. While she generally puts forward a facade of evilness, she genuinely cares about her friends and will do her best to help them.
Voiced by: Aya Uchida
Shiho is the dominant personality, and is aware of everything occurs when Shirou is in control of her body. While Shirou has a crush on Youko and attempts to discover if Asahi is aware of her secret, Shiho is more playful and amorous, deriving enjoyment from being perverse. She often tries to force Youko into admitting her true feelings towards Asahi, and having a perverted crush on Asahi, and tries to expose herself to him. While her father is also a wolfman, her mother is only shown in silhouette and is referred to as the "Charismatic Female Pervert."
Voiced by: Kenichi Miya
Shishido is Youko's childhood friend and a wolfman. Initially appearing as Shirou, an intimidating man with sharp teeth, upon seeing any depiction of The Moon, he transforms into Shiho, a busty, self-proclaimed pervert. A second glimpse of The Moon transforms Shishido back into Shirou. Shiho is the dominant personality, and is aware of everything occurs when Shirou is in control of her body. While Shirou has a crush on Youko and attempts to discover if Asahi is aware of her secret, Shiho is more playful and amorous, deriving enjoyment from being perverse. She often tries to force Youko into admitting her true feelings towards Asahi, and having a perverted crush on Asahi, and tries to expose herself to him. While her father is also a wolfman, her mother is only shown in silhouette and is referred to as the "Charismatic Female Pervert."
Voiced by: Mao Ichimichi
Principal of Asahi's high school and a thousand year-old demon, although her appearance is of the body of a young girl with demonic horns. Despite her age, she is often quick to exhibit childish tendencies, such as playing large-scale pranks on the people at school and quickly getting excited whenever she believes sweets are nearby. She takes special delight in ruining Akari's life when given the chance. As a demon, Akane is capable of magically transforming herself and others around her, as well as multiplying herself (though the clones will also display copies of her horns on their head). She was the teacher of Youko's parents when they attended her high school. It is implied her present form is not her true appearance. In the past, it was revealed she had a much more mature and menacing appearance. Her true appearance is never confirmed. She is extremely powerful, although not omnipotent, ranking among the other high-rank demons, but she is jealous of them, since she does not have any legends about herself. 
Voiced by: Emi Nitta
Asahi's homeroom teacher and former delinquent known as "Akari of the Hundred Visits" due to responding violently after being rejected by, apparently, 100 boys across different schools. She is Akane's great-great-granddaughter, and while she does not retain any demonic powers, her short temper and unnaturally strong fighting ability allows her to rein in Akane's misbehavior. Aware of the various secret students in her class, she attempts to be a reliable authority figure. However, she often finds herself dragged into crazy situations between her heavy drinking, her near-impossible wish for a perfect man to literally appear on a white horse and marry her, and Akane's own devilish mischief. She is later called Akari-chan by Youko, who comes to see her as an equal rather than just a teacher. She works hard to change her image of a spinster into a cute girl, but her attempts are always marred by Akane's pranks. 
Rin is a first high school student and secretly Asahi's and Mikan's time traveling granddaughter from the future (The daughter of Asahi's Daughter and Mikan's Son). She claims to have returned to the past to prevent a dystopian future in which an entity known only as "Charismatic Female Pervert II" has taken over the world, turning 90% of the male humans on Earth into masochistic slaves and 70% of the females into like-minded perverts - however, this is later revealed to be a lie, as she has actually returned to try and prevent the series of events leading to Youko being exposed and abandoning Asahi. She travels back in time using a dragon-shaped time machine and wields a sword capable of making clothes more modest. Usually quiet, she is protective of Asahi, whom she treats like a grandfather, not adapting her behavior to his younger appearance. Also like Asahi, she has trouble keeping a secret, accidentally letting slip letting him know she was a time-traveler despite the possibility of creating an irreversible temporal paradox by doing so. Her grandparents are eventually revealed to be Asahi, Yōko, Mikan and Okada.
Voiced by: Daisuke Hirakawa, Wataru Hatano and Hiro Shimono
Asahi's three male friends. Okada wears glasses and is often seen with a juice box in his mouth. Shimada often attempts to hit on various female classmates (notably Nagisa's disguised older brother, Ryō) with little success. Sakurada is younger than he appears to be, and is often smiling. All three try to support Asahi, though they often mock his inability to keep a secret. Throughout the manga, their first names have been revealed; Sakurada's name is Kōsuke, Shimada's name is Yūta, and Okada's name is Kanade.
Okada is revealed to be a read people's minds, against his own wishes. He became friends with Asahi and Mikan, when he was amused by Asahi's inability to hide what he was thinking and Mikan's contrasting actions and personality. He gradually fell in love with her, but refrained from confessing as he was aware of Mikan's feelings for Asahi. He eventually did confess, and it is revealed they got married in the future.
Sakurada was once a delinquent, against his own wishes. He was often attacked and mocked because of his naturally massive build and menacing appearance, and no one would believe him. Before he entered high-school, he and his gang attacked Akari, but she beat them all senseless. She consoled Sakurada, which resulted in him falling for her. He eventually succeeded in confessing to her and they began to date in "secret", while everyone actually knew the truth.
Voiced by: Rikiya Koyama (Genjirō), Yoko Hikasa (Tōko)
Youko's father and mother who had a high school romance similar to Youko and Asahi. Genjirou is a scarred 4-meter tall vampire with an intimidating aura. He is overly protective of his daughter, at one point even disguising himself as a female assistant teacher at the school to find