Wataru Matoba is a first-year literature student at Kamogawa Music College. He plays bass and writes lyrics for the band Argonavis, a name he also chose. His father worked as a mariner, and his mother, a nurse, was often absent. Consequently, he spent much of his childhood with his older brother, Kenta Satozuka. Observing Kenta's band involvement inspired Wataru to take up the bass, but it also fostered an intense inferiority complex toward his brother, fueling resentment and a drive to surpass him.

Personality-wise, he presents a stark contrast to Argonavis's enthusiastic leader, Yuto. Wataru tends to be cautious, realistic, and pessimistic, frequently pointing out potential negatives. This stems from deep concern for the band's success, not opposition to Yuto's ideas. Despite his reserved nature, he cares profoundly about the band's future. Academically, he shares classes with Yuto and others in the Faculty of Literature and enjoys reading.

Physically, he has auburn hair cut to ear length and gray eyes, standing 175 cm tall and weighing 64 kg. Within the band, he serves as a grounding force, tempering Yuto's idealism with practical considerations. During the band's participation in Tokyo's LRFes, he voiced concerns about their readiness for such a large-scale event while remaining dedicated to their goals. His role in the Four-Temperament Ensemble is melancholic, reflecting his analytical and sometimes somber outlook.

His relationship with Kenta remains complex. Despite reconnecting through music, Wataru struggles with lingering feelings of inadequacy and finds it difficult to be emotionally open. This dynamic shapes his cautious approach within Argonavis as he strives to prove his own worth independently.