Mahiru Kōzuki is a sixteen-year-old high school student and the main protagonist of Jellyfish Can't Swim in the Night. She is a talented illustrator who once worked under the name Yoru Umitsuki, but stopped pursuing art after a painful experience in her childhood. As a young girl, she painted a mural of a jellyfish on a wall in Shibuya, only to have her friends mock the drawing, calling it weird and creepy. This event deeply wounded her and caused her to abandon her passion for drawing. From that point on, she adopted a conflict-avoidant personality, determined to live as an ordinary, unremarkable girl who does not stand out or invite judgment from others. She is described as a mass-produced type of high school girl who prioritizes avoiding trouble and fitting in with her peers.

Despite her efforts to blend in, Mahiru possesses a highly sensitive and caring nature. She is able to empathize deeply with others, but this same sensitivity makes her easily hurt by even small criticisms or thoughtless remarks. Beneath her surface-level desire for normality, she also harbors a quiet wish to become someone other than who she is. Her personality is a mix of outward agreeableness and an inner stubbornness, as well as a hidden competitive streak. She is good at reading social situations and accommodating those around her, but she often suppresses her own true feelings to maintain harmony. Her childhood friend, Kiui Watase, has described her as troublesome for this very reason.

The trajectory of her life changes dramatically when she encounters Kano Yamanouchi, a former idol who is now an anonymous singer. Kano recognizes Mahiru as the creator of the jellyfish mural that she had admired for years. Through Kano’s passionate encouragement, Mahiru is inspired to pick up her pen again and begins creating illustrations for Kano’s anonymous artist group, JELEE. This marks the start of her return to the world of art. Within the group, she is the only member who is not directly involved with music, as her role is strictly the illustrator. Her daily life includes not only her art but also part-time work with Kano at a karaoke bar.

Mahiru’s most important relationship is with Kano, who acts as both a muse and a catalyst for her growth. Kano’s unwavering belief in Mahiru’s talent gives her the confidence she lost years ago. They are also joined by Kiui, who is not only her childhood friend but also a popular VTuber named Nox Ryugasaki. Despite attending different high schools and not seeing each other in person for two years, they maintain a close bond through online communication. Kiui is one of the only people with whom Mahiru can speak openly about her past without fear of judgment. Other key relationships include her younger sister, Kaho, who is curious about streaming and video, and later, a talented pianist and composer named Mei Kim Anouk Takanashi, who becomes the group's composer.

Throughout the story, Mahiru undergoes significant development. She starts as a girl paralyzed by the fear of ridicule, who hides her special talent to live a simple, unnoticed life. However, as JELEE’s activities progress, she gradually learns to embrace her identity as an artist. Her arc culminates in her reclaiming her old alias, Yoru Umitsuki, and finally creating art for herself rather than for the approval of others. She shows remarkable growth by making brave proposals for the group and learning to express her own opinions, eventually standing as an equal counterpart to the more outwardly forceful Kano.

In terms of abilities, Mahiru is a skilled illustrator whose art forms the visual identity of JELEE. Her drawings, particularly of jellyfish, are central to the group’s aesthetic. Beyond her artistic talent, she is also noted to be good at cooking and cleaning. Though not a performer, she once participated nervously in a live show by playing the triangle. She also has a fondness for sea creatures, especially jellyfish, which serve as a recurring motif in her life and work.