Akino Sakura, known by the nickname Cosmos, is a main character in the story and a third-year student at Nishikizuta High School, where she serves as the student council president. Her nickname derives from a play on her name's kanji; removing the character for "field" from her surname transforms it into another word for the cosmos flower, often read as "akizakura". She was born on September 19 and has A blood type. Physically, she is recognized throughout the school for her striking beauty, with light-purple hair often styled with small flower-shaped clips near her ears, darkish-purple eyes, and a mature figure, which includes a distinctive mole beneath one eye.

Beneath her composed and elegant exterior, Cosmos possesses a personality that is more approachable and unexpectedly childlike than her cool reputation suggests. She is a hardworking, proper, and exceptionally organized individual who values punctuality, honesty, and patience. Her constant companion is a pink notebook, which she uses to meticulously write her schedule, make notes, and plan for various future scenarios. However, this strength in planning is also her greatest weakness; she has very little ability to improvise. When caught off guard, made extremely nervous, or faced with the unexpected, she becomes flustered, loses her composure, and can speak awkwardly. This nervousness sometimes manifests in quirky verbal tics, such as unintentionally talking like a cat-girl or slipping into an old-fashioned Edo-period samurai dialect. In her private life, she has a girlish and somewhat innocent side, enjoying cooking, handicrafts, reading shoujo manga, and even still believing in Santa Claus. She also loves dogs and is particularly fond of cuddling with her pet, Big. Her tastes in fiction lean toward romantic stories, and her favorite song is the first opening theme for the anime "Kimi ni Todoke".

Cosmos comes from a privileged background; her father is a hospital director, and she herself aspires to attend medical school. Her academic performance is flawless, as she consistently holds the top rank in her grade year. Initially, her primary motivation in the story is her romantic interest in Taiyo Oga, a charismatic baseball player and the protagonist's best friend. In this pursuit, she views her underclassman, Amatsuyu Kisaragi, as a reliable and "cute junior" who can act as a useful assistant or tool to help her win Taiyo's heart. Despite her high status and popularity, she secretly feels a degree of loneliness, as her position and studious nature have prevented her from having typical social experiences like going to karaoke or festivals with friends.

Cosmos plays a central role in the story's early conflicts. After she discovers that Taiyo is in love with another girl, Sumireko Sanshokuin, she feels profoundly betrayed upon believing that Amatsuyu kept this secret from her. In her anger, she harshly condemns him as cruel and insensitive, expelling him from the student council. This reaction stems from her deep-seated hatred for deception and dishonesty in others. Her character development truly begins when she realizes that the entire situation was a manipulation orchestrated by Taiyo. Filled with regret for her harsh treatment of Amatsuyu, she works to make amends and stop the harassment against him. Through this experience, her feelings evolve from a superficial romantic ideal to a genuine appreciation for authentic connection. Her initial infatuation with Taiyo fades, and she eventually develops sincere romantic feelings for Amatsuyu, entering into a respectful rivalry with Sumireko. Her abilities are less about physical prowess and more about intellect and influence. She is a brilliant strategist and a master of organization and deductive reasoning, using her position as student council president to investigate schemes and uncover the truth, as she does when exposing the plots of other characters.