Seiya Mizuno (水野 星夜 Mizuno Seiya) is the son of a very rich family, and trained professionally in culinary arts.
Seiya is quite tall and slim. He has short blonde/light colored hair like Sora's and clear eyes. He is noted as very handsome and he looks just like Sora, so much so that Najika and even Daichi confused him as Sora when first meeting him. Seiya has a lean stature and thin fingers, good for cooking.
Seiya is conceited and very confident in his cooking abilities, as he has been professionally trained in the culinary arts since he was extremely young. Also, his pampered upbringing serves to make him cocky. However, it is shown that beneath his high and mighty facade, Seiya is actually quite caring and hardworking.
He is very conceited on the outlook, but it is shown that he tries extremely hard to please his father. From youth, he was constantly ignored by his parents, who were always busy. Seiya likes very much to win, and is determined to be the best at cooking. At first, he believes that having skills and high-end ingredients are absolutely necessary to excellent cooking, but is shown otherwise by Najika, who he develops a crush. In the end, he and Najika agree on being supportive friends and in a bonus chapter, Seiya begins dating Akane.
Since he was young, there were many expectations on Seiya by his father. He trained hard to be a top-end chef, with his father telling him that if he wasn't the best, he was no good. Seiya became somewhat insecure and unable to comprehend why others could bear not being first. When he saw Najika making an "ugly" cake as a kid, he was very confused, thinking that everything needed to be professional.
When Seiya grew up, he started attending Seiko Academy. The director brought Seiya to replace Najika. At first, Seiya is confident in his abilities to crush Najika's cooking skills, but he soon realizes otherwise. Determined to be the best, Seiya hosts a cooking competition in front of professional judges, and he ties with Najika, but he is still disappointed. Nevertheless, after deciding to work at Fujita Diner to figure out why Najika's cooking is so good, he realizes that good ingredients and fancy equipment aren't everything and his personality and approach takes a turn for the better.
Seiya sees Najika as inferior at first, but soon realizes that her cooking is more aesthetically pleasing, despite being cheaper. This infuriates him, but Najika helps him see beyond high-quality ingredients and equipment, and his cooking style changes. After working at Fujita Diner to find out why Najika's food is more loved than his, Seiya grows to fall in love with her, being very outright in his advances. Najika is, at first, quite flustered, and a little bit confused. When she eats his flan, she believes him to be her flan prince, because while he did not deliver her the flan, he was the one that made it (Daichi was a guest at the hotel his family owned). Believing him to be her flan prince, Najika tells herself for a while that Seiya is the right guy, but later she confesses and says she can only see him as a friend. In response, Seiya tells her she has become his greatest rival and cooking friend.
The two settle with being good friends, pushing each other in their cooking capabilities. At first believed by everyone to be the ideal couple (since both were excellent chefs and Seiya publically romanticized Najika), they decide to remain friends. While Seiya may be conceited towards Najika at first, the two make up.
Seiya finds Akane quite annoying at first, stating that she was cute and charming, his type of girl, but that her personality stank. Akane stubbornly finds Seiya a big airhead, and the two frequently argue. However, they grow to be good friends, that bicker constantly but support one another. Later, in the bonus chapter, "Akane and the Black Sesame Pudding," it is shown that Akane likes Seiya, and that the two end up dating.
While they are not friendly to each other at first, the two grow in friendship and eventually, love. Seiya constantly teases Akane about being noisy, saying that she will not find a boyfriend like that. Ironically, Seiya develops feelings for her, and they start dating at the end of the manga. Their relationship is half comic relief and half seriously involved, but overall, they grow to become close and boyfriend/girlfriend.
At first, Seiya clashes with Daichi, because of his bad attitude towards Najika. When Seiya tells Najika of his feelings for her, he is sometimes exasperated at her liking Daichi. Nevertheless, the two evolve from being rivals for Najika's feelings to becoming friends.