He is first seen in the beginning of episode 5, where he is in a Deal. His Asset is a robot that has a triangular form, named Gassa. He seems to be able to synch with Gassa, allowing him to see where an opponent's attacks will strike, allowing him to then easily dodge them, effectively guarding his wealth while allowing the opponent to use up money. He can also attack with Gassa, using it as an accessory to fly and deliver a decisive blow to the opposing Entre just as time starts to run out.
Additionally, he seems to be one of the less mentally stable members of the Starling Guild.
When Japan faces a second collision with the "C" wave spreading the globe, while Jennifer Satou challenges Mikuni to a Deal, Kimimaro challenges Horii to one as part of a double-team effort to drain Mikuni of his wealth in order to save Japan's future. Because Kimimaro wanted to spare Msyu harm in her weakened state, Horii quickly got the upper hand in dealing damage, showing a sadist mentality towards Kimimaro, who he saw as a traitor to the Starling Guild. It was only when Gassa experienced a damaging error that Kimimaro was able to counter-strike with a devastating blow that left Gassa and its card unfunctional. In frustration, Horii kicks Gassa, which then comes to life and reattaches itself to Horii. Horii and Gassa unleashed a powerful strike against Kimimaro, but Kimimaro finally calls out Msyu, who protects him in time. Horii and Gassa prepare a stronger attack, but Kimimaro and Msyu unleash her Macro-level attack, which completely depletes Horii's remaining wealth. Having gone bankrupt, Horii is instantly banished, and vanishes from the Financial District.
