Onsen is a character introduced in the Duel Masters Victory anime series, where he serves as an opponent of the young protagonist Katta Kirifuda and his ally Dragon Ryuu. He is the leader of the Shachihoko Group, and his companions Rikishi and Shachihoko operate alongside him. His name means hot springs, and that theme runs through both his identity and his approach to dueling. He is described as a man who carries the heat of fire and the coldness of water, using cards from the Fire and Water civilizations to reflect that dual nature. He has a strong fondness for hot springs, and he also has the ability to change his appearance dramatically.

In the story, Onsen belongs to the alien group that stands against Katta and his friends. One of his notable appearances takes place when Katta sets out to recover a card stolen from his friend Benchan. The trail leads Katta to a public bathhouse called Heiyuu, where Onsen is waiting, and the encounter establishes him as a striking and capable antagonist. His role later grows when he becomes a tournament rival. After witnessing Katta's fierce battles, Dragon Ryuu faces Onsen in the semifinals, and the match is framed as a clash between two duelists with an established rivalry, implying that their conflict predates the tournament.

His abilities are built around the combination of Fire and Water, allowing him to wield both aggressive heat and disciplined cold in his duels, and he is also a strong leader who commands the loyalty of his group. As a character, he is theatrical, self-assured, and tightly connected to the hot springs motif, and his recurring confrontations with the protagonists make him a persistent obstacle for both Katta and Dragon throughout the Victory season.