Who is Lieutenant Paul Duncan from Deal Breaker and what is their importance?

Paul Duncan is a former homicide investigator and was Adam Culver's best friend. Duncan has been a friend of the family for years, and Culver's children refer to him as "Uncle Paul." Prior to Adam Culver's death, Culver learned that Duncan and his wife Carol had been having an affair for six years. Physically, Duncan is described as short and stocky with salt-and-pepper hair. Duncan is probably around sixty years old.