Chiaki Yoshino (吉野千秋, Yoshino Chiaki) is a main character in the Sekai-ichi Hatsukoi series and a central character in the light novel series The Case of Chiaki Yoshino.
Chiaki is a popular shoujo mangaka for Emerald magazine. He writes under the female pen name Chiharu Yoshikawa (吉川千春, Yoshikawa Chiharu) since he thinks it would disappoint his female audience to find out his stories are written by a guy.
Chiaki has medium length brown hair with side bangs that cover his left brow and blue eyes. He is usually underweight, due to lack of nutrition and sleep, when frantically trying to meet a deadline. He wears casual clothing most of the time and is only seen in a suit during the Marukawa's New Year's party. Like most male characters in the series, Chiaki is tall for Japanese men, more than likely in the slightly above average height range. However, Chiaki is significantly shorter than his lover.
Hatori is Chiaki's current lover, editor and best friend. Hatori and Chiaki have known one another their entire lives, since even in their elementary school years. They have always been very close to where Yuu remarks that Chiaki and Hatori "have always been joined at the hip". When Chiaki was struggling with balancing both his college career and drawing manga, Hatori assured him that he will take care of him so he would not have to worry. Chiaki later learns that Hatori joined Marukawa in order to become his editor and continue to remain close to him.
One night, however, Hatori made his feelings clear to Chiaki when he abruptly kisses the latter while he slept, resulting in them having a make out session. After this, Chiaki became very confused about his feelings since he had been under the false impression that Hatori was dating Yuu, his other best friend, and having a lovers spat - feeling like a replacement. When he briefly thinks to himself how he wished Hatori was in love with him instead of Yuu, his confusion about his feelings for Hatori become even greater.
However, Hatori later contacted him for a private meeting in order to talk. After meeting, Hatori tells Chiaki that he has been in love with him for his entire life - clearing up that he and Yuu were not dating and the two actually hate each other. Hatori further tells him he never admitted to his true feelings because he felt that Chiaki would be disgusted by another man having feelings for him. However, Chiaki realizes in surprise that he isn't bothered by it at all. Hatori then declared that since he has betrayed Chiaki's trust, he felt it was best to leave his life. Panicking since he knew, in the least, that he didn't with Hatori to leave, Chiaki abruptly demanded that Hatori kiss him. He told Hatori that if he was truly not okay with even kissing then he could leave; wanting to know if they could make a relationship work in order to keep Hatori near him. Hatori complied and, when seeing kissing was okay with him, Chiaki begins a relationship with Hatori.
Slowly after this, their relationship grew and Chiaki begins to slowly realize that he loves Hatori more than as his best friend. This is seen when Hatori believes that the relationship is not genuine and that Chiaki only mistook feelings of friendship for love and to keep him by his side out of desperation. When Hatori begins to break up with him, Chiaki becomes uncharacteristically angry and yells at Hatori to not make assumptions about his feelings. Chiaki goes onto say that he could never date or become sexually intimate with a guy out of sympathy or compassion, even revealing that the only man he gets "excited" by is Hatori. After confessing all this, Chiaki makes his first step in gradually realizing the true extent of his feelings.
Chiaki's growing feelings are further demonstrated when he experiences jealousy. In one instance, when seeing Hatori with an ex-girlfriend who was becoming flirtatious, this greatly angers him and brings him to go to Yuu's house to drown his feelings in alcohol. His feelings are just about officially confirmed when Yuu begins forcing himself onto Chiaki and kissing him against his will, saying how he could maybe fall in love with him by sleeping with him. Chiaki responded by punching him and yelling that he could never be with him like that. A shocked and dejected Yuu demanded to know why Hatori was the only one "good enough". Chiaki says he doesn't know, but that Hatori is simply the only man he can be intimate with. Yuu responds by saying that "[Chiaki] knows why", making him think.
After this, Chiaki begins to realize that since he can't bear seeing Hatori with someone else romantically, then that is an indication that he must love him. When Hatori explains why he was with his ex-girlfriend, she is a co-worker he simply ran into, Chiaki becomes embarrased for his furious reaction. Hatori takes this as an indirect confession of love for him and the two make up. As of season two, in the anime, Hatori and Chiaki have been together for a year.
Yuu met Chiaki in Junior High and became close friends with him after finding out that they shared an interest in the same books and enjoyed the same entertainment. Yuu and Chiaki's friendship had always made Hatori jealous, especially since Yuu is always teasing him about his feelings for Chiaki and is clearly close to Chiaki. He also frequently insults and criticizes Hatori's household abilities and his home cooked meals. Hatori is very aware of Yuu's feelings towards Chiaki, as the other way around, and because of this the two act very cold towards each other. However, they do their best to maintain an illusion of a polite relationship for Chiaki's sake, who wants them to get along. Chiaki is very naive and oblivious of Yuu's feelings until the latter confessed to him blatantly about his feelings.
Chiaki, wanting to clarify how he felt about Yuu and Hatori, thought and imagined how he would feel if he kissed them both. While he blushed and gasped in strong emotion when he thought about kissing Hatori, the thought of kissing Yuu made him greatly uncomfortable and almost disgusted. Despite Yuu confessing to him, Chiaki tries to think of the moment as a joke since he still valued their friendship.
However, Chiaki later makes it bluntly clear that he doesn't wish to become romantically intimate with Yuu whatsoever. When Yuu began forcing himself onto him, with hopes Chiaki could love him after becoming intimate, Chiaki responded by punching him after ignoring his protests to stop. Dejected and heartbroken, Yuu broke down in tears and demanded to know why only Hatori is "good enough". Chiaki explained how he doesn't know, but that's just how it is. Yuu, however, remarked that the answer was clear. After this, the two still remain good friends although Yuu continues to cling to his hopes that Chiaki will one day become his.
Yuu is known to enjoy drawing Chiaki in any position he casually finds himself in. Yuu is always bugging Chiaki to let him draw him naked, which makes Chiaki uncomfortable although he dismisses the frequent request as a joke. Yuu became a manga assistant so that he could stay close to Chiaki and is known to be the best assistant on the team and gets the most work done in a brief time period. When he had took a brief absence from the job, the other fellow manga assistants were lost without him and wouldn't have been able to turn in their work on time if he hadn't showed up later on, much to Chiaki's relief.
Chinatsu is Chiaki's sister. Just like regular siblings, they get into many fights and annoy each other. However, they still love each other very much. Chiaki is aware that Chinatsu has feelings for Hatori by observing how she acts around him which, as seen in the anime, makes him jealous.
In an anime OVA, Chiaki is just about forced to visit home to see his mother and sister, who have missed him, and brings Hatori along with him since his mother is known to favor him. When his mother makes a comment about how good a couple Chinatsu and Hatori would make, this causes him to spit out his drink in near horror. Angered by this, as well as his sister's clear affection for Hatori, Chiaki ends up leaving in a huff. Chinatsu doesn't know about Chiaki and Hatori's true relationship as lovers.
- Chiaki is one of the most popular Mangaka at Marukawa Publishing. Also, only a few of the staff members are aware that Yoshikawa Chiharu is male.
- Chiaki loves to eat sweet food.
- Chiaki has a king-sized bed, which can accommodate five people. Ironically, he prefers to sleep in Hatori's small size bed instead, which can barely hold two people.
- Chiaki cannot cook, including simple breakfast meals.
- His relationship with Hatori is referred to as "Domestica" by the fans, referring to how Hatori takes care of Chiaki and is considered the most "domestic" of the three main pairings.