Ritsu is a 25-year-old jaded literary editor. He is the only son of the head of Onodera Publication and previously worked at his father's company in the literature department. Hearing his fellow co-workers accusing him of taking hand-outs from his father, he decides to show that he can be successful on his own by going to work at Marukawa Publishing. After being hurt by his first love in high school when he was 15, Ritsu vowed to never fall in love again. His resolve is tested, however; when he is reunited with the boy he once loved: now very much a man and his new boss things get very complicated for him. And now he must figure out his feelings while also juggling his career and seduction from his former first love and new boss. It was also mentioned that he was one of Usami Akihiko's (from Junjo Romantica) editors at his previous job.
Formerly known as Saga Masamune, before his parents divorced in high school. He is the editor-in-chief of the Emerald department at Marukawa, and is sometimes referred to as "the Capable". He and Ritsu were once in a brief relationship in high school but split up over a misunderstanding, where they both thought each other was wrong. It is revealed that he later had a break down from the break up, family problems, and other personal issues, which in turn left him with an unrequited love for Ritsu. When both he and Ritsu realize who each other are, he promises Ritsu he will make him fall in love with him again much to Ritsu's contempt. In Sekai Ichi Hatsukoi: Takano Masamune no Bai (lit. Takano Masamune's Case), Takano's perspective of his and Ritsu's first meeting and romantic development and previously unknown events are shown.
Ritsu's childhood friend and his fiancée, due of an agreement their parents had when they were kids. Sweet, polite and sensitive, An has deep feelings towards him, despite of being rejected by Ritsu (whom she calls 'Ricchan') when she confessed her love; despite of figuring out about the bond between Masamune and Ritsu and accepting it, she still hopes to gain Ritsu's affections someday.
Yoshino Chiaki, is a 28 year old shōjo manga artist known as Yoshikawa Chiharu. His editor in-charge is Yoshiyuki Hatori, who is also his childhood friend. His two best friends are Hatori and Yanase, but the lines between friendship and love begin to blur after a number of confusing incidents. Chiaki begins to question his feelings for Hatori after not only witnessing him and Yanase together in what he thinks is a kiss but Hatori kisses him one night. Chiaki begins to realize he has to confront his true feelings or risk losing them both.
Hatori, also known as Tori, is 28 years old and an editor in the Emerald department. Although in the beginning he is serious and reserved, we later learn that he has harbored both deep and secret feelings for his childhood friend Chiaki. After kissing him and confessing his love, Hatori pulls away fearing that Chiaki doesn't feel the same, but instead loves his rival, Yanase. Although Chiaki does somewhat have the same feelings, unbeknownst to Tori.
Chief assistant for mangaka Yoshino Chiaki. Hatori and Chiaki both came to know Yuu when they were in junior high. An immediate tension is visible between Hatori and Yanase. It isn't until later that it is discovered that both are vying for the love of Chiaki
Kisa is a fellow editor in the Emerald department, and because of his "baby face" he is mistaken for being much younger than his true age of 30. He's never been in a serious relationship and doesn't think himself capable of being in one because of his tendency to fall in love for only good looks without knowing a guy's personality. Because of this, he immediately questions his feelings for Kou and doesn't hold out much hope for a relationship with him. His mentality can be described by his line: "love at first sight works well enough in a manga, but this is reality."
Yukina is a 21-year-old clerk in charge of the shōjo manga section at the bookstore, Marimo books. He's been aware of Kisa coming in to watch him, but when he finds out Kisa is the editor behind all of his favorite works he begins to develop feelings for him. He's willing to fight to make Kisa understand that what they feel for each other is real. And that Kisa is able to actually fall in love.
Yokozawa works in the Financial department at Marukawa. He is friends with and had a physical relationship with Takano when they were both in college. It's also common knowledge in the publishing world that he helped Takano get into publishing and get his position at Marukawa. He is seriously jealous of and resents Ritsu, blaming him for "screwing up" Takano. He often warns Ritsu away from Takano, even going as far as proclaiming Takano is his. He has his own side story called Takafumi Yokozawa No Baii(Takafumi Yokozawa Case) about his new love interest, Kirishima, and him handling his rejection from Takano
He was first introduced in Junjo Romantica as the editor in chief of Japun and is now Yokozawa's new love interest. He is raising his daughter, Hiyori, on his own due to his wife passing away a few years ago. After blackmailing Yokozawa, the two start dating once they realizing their feeling for each other. He has a habit of teasing Yokozawa calling him the mother of his daughter Hiyori and his family.
Hiyori is the only child of Zen; 10 years old and lost her mother time ago. She bonds quickly with Yokozawa, to whom she refers as 'Onii-san' (big brother). She's responsible for cooking her father's meals and is very fond of kittens and green tea sweets
Mino is a fellow editor in the Emerald department, who is constantly smiling through all the stress and exhaustion. It has been hinted that he has a dark side, with Takano saying "Make sure you don't make Mino upset".
Isaka is the senior managing director of Marukawa Publishing, and is acquainted with Ritsu's parents. He is quite the mysterious man. He is also a character of Junjo Romantica.
Off & On