Yamamoto is Kouta's partner on the police force. He has known Kouta since he was in High School and even took him into custody before. He knew Kouta's father rather well. Yamamoto knows about Kako and Kouta's marriage, and even gave Kako information about Kouta before he proposed. He has fun lightly teasing Kouta about the situation and reminiscing about when Kouta was younger, as well as is extremely friendly to Kako.