Nanjo is Kinzo's personal physician and close friend. He is also Kinzo's opponent in chess. (from wikipedia)

Spoiler

He met Kinzo a long time ago and would come to be friends with him for decades. Requiem of the Golden Witch reveals that their first meeting was during World War II. A young Nanjo first met Kinzo when he brought Beatrice Castiglioni to his clinic in secret after they escaped the Japanese military base on Rokkenjima. Nanjo was given a gold bar in exchange for secrecy and would become one of Kinzo's oldest friends. He also tended to the baby Yasuda and helped them survive their fall from the cliff incident. The Twilight of the Golden Witch manga expands on this scene, showing that Nanjo had additionally performed surgery on Yasu to remove their extra toes from their polydactyly to conceal their relation to Kinzo and was directly responsible for them becoming "furniture", wanting Yasu to live a happy life even if they couldn't have children.