He was Ken's partner during their adventure in the Digital World. After Ken was implanted with a Dark Spore, unaware of the consequences, Ryo took care of Ken before getting him back to the Real World.
Though Ryo holds nothing against Rika, their relationship is somewhat strained due to her pride, as she lost to him in the card game tournament. In the American dub of Digimon Tamers, Ryo tries to impress and even flirt with her, giving her nicknames like "Pumpkin" or "Wildcat".
Kazu is in awe of Ryo due to his status as the Digimon King. During the battle against ADR-06: Horn Striker, the two work together and Kazu's Guardromon even saves Justimon's life earning Kazu Ryo's thanks, causing him to be very happy.