Mondo is a ninja warrior and spy from the feared Yaki Clan, a secret clan of ninjas that has operated throughout history as hired agents for various powers. In modern times, the clan makes its home in the forbidden forests of Aoigahara at the base of Mount Fuji, and Mondo serves as its leader and strongest warrior. He enters the first Battle Arena Toshinden tournament under orders from a client to gather intelligence on the Secret Society. After completing that mission, he receives an invitation to the second tournament; his client orders him to decline, but Mondo ignores the restraint and participates of his own accord, suggesting a personal motive beyond mere employment. Following the fall of the Secret Society, he competes in the third tournament hosted by the Organization.

Mondo carries the Seiryu Spear, an heirloom passed down to the strongest warrior of the Yaki Clan and one of the Four Sacred Arms. He is a master of the naginata, fighting with long‑range strikes and powerful, hard‑hitting techniques. His combat style includes projectile attacks, such as spinning his spear to launch fireballs, as well as anti‑air and lunging thrusts. While his attacks deal high damage, they often have notable openings that require careful timing. He is described as emotionless and stoic, typical of a seasoned spy and ninja.

Key relationships include a rivalry and acquaintance with Fo Fai, the elderly magician who hides a murderous nature. Later, after Eiji Shinjo becomes head of the Gerard Foundation and learns of the Four Sacred Arms, he begins hunting Mondo to claim the Seiryu Spear. Genma also seeks Mondo for the same reason. It is believed that before the fourth tournament, Eiji succeeds in capturing Mondo, as one of Eiji’s fighters, Eos, wields the spear thereafter.

Throughout the series, Mondo evolves from a covert operative on assignment into a targeted survivor, his own agenda and warrior pride driving him to continue fighting despite mounting danger.