Kazuna Masunaga leads the idol unit MooNs within B-Project, though reluctantly. He has honey-blond hair parted to the right, turquoise eyes, and wears two earrings on his right ear. Light-colored sweaters or shirts are his typical attire.

Born in Fukuoka, his childhood involved his mother's early abandonment and his father's frequent work absences. His older brother, Nozomi—six years his senior—became his primary caregiver, and Kazuna idolized him. An unresolved longing for his mother drove him; he pursued idol work hoping she might see and recognize him publicly. He later moved to Tokyo for his career and feels insecure about his native Hakata dialect.

Kazuna joined the training agency Bambi after a Fukuoka audition, catching the attention of Bambi representative Atsushi. When his father couldn't accompany him, Atsushi personally drove to Fukuoka to bring him to Tokyo. He spent two years at Bambi before MooNs formed. During this time, he co-hosted the "Prodigy Brunch Special" variety show with Tomohisa Kitakado, forging an early friendship.

Personality-wise, Kazuna embodies the diligent, cooperative, and gentle honor student archetype, often mediating within his group. Underneath lies competitiveness and a pronounced jealous streak. He harbors deep-seated insecurities and a persistent sense of inferiority, particularly towards Tomohisa, whom he perceives as naturally surpassing his efforts. This manifests as emotional distancing, though Tomohisa remains admiring and unaware. A perfectionist, Kazuna habitually shoulders burdens alone, leading to self-imposed pressure and occasional emotional clouding. His profile suggests INFJ traits with an Enneagram Type 3w4, reflecting an achievement-oriented mindset combined with a desire for authentic connection.

His relationships are multifaceted:
* With **Tomohisa**, their bond deteriorates due to Kazuna's inferiority complex but gradually mends through communication. Tomohisa viewed Kazuna as an older brother figure, symbolized by matching bracelets gifted by Tomohisa's sister during their Bambi days.
* With **Tsubasa Sumisora** (B-Project's A&R manager), he shares mutual respect and trust. While prioritizing his career over romance, he displays growing affection—seeking alone time with her during Christmas shopping, blushing at her compliments, and inviting her to meet Nozomi for support. He later states he wouldn't oppose dating her to understand her better.
* With **Momotaro**, he commands deep respect and serves as a calming influence, though Momotaro worries about his reluctance to rely on others.
* With **Nozomi**, their relationship fractures over time. Repeated meeting cancellations dishearten Kazuna. In a critical confrontation, Nozomi falsely announces his engagement and demands Kazuna quit idol work to avoid scandal. This triggers an emotional breakdown where Kazuna accuses Nozomi of embedding resentment in his childhood, prompting Nozomi to counter that Kazuna's obsession with finding their mother overshadowed their bond. This clash forces Kazuna to reevaluate his motivations.

Kazuna's development centers on evolving self-worth and purpose. Initially driven by the hope of reuniting with his mother, he later finds fulfillment through connections with MooNs, B-Project, and their fans. Reading fan messages during a crisis instigated by Nozomi's demands solidifies his commitment to bringing joy to supporters, shifting from seeking external validation to embracing intrinsic value in his work and relationships.