Frank Belson is a sergeant in Boston Homicide. He's one of Spenser's oldest friends in the series, apparently from Spenser's days with the Suffolk County DA's office (The Godwulf Manuscript). He remains one of Spenser's most reliable contacts in the police department throughout the series.
Belson is a "thin, hard" man with a perpetual five o'clock shadow. He's fond of cheap cigars which smell terrible. He is a skilled detective with a remarkable attention to detail; Spenser mentions more than once that Belson "misses nothing" when he takes in a crime scene, memorizing every aspect of it. Despite his skill, Belson remains a sergeant for the duration of the series - at least a few decades. This may be because he refuses to take the lieutenant's exam.
In Thin Air we learn that Belson was unhappily married for over 20 years to a woman named Kitty, who he eventually divorced. He had two daughters with Kitty. By Thin Air Belson had remarried, to a younger woman named Lisa St. Claire (AKA Angela Richard). He has a wry sense of humor and a fierce sense of justice.
Belson frequently provides Spenser with information (license plate searches, police files, etc.) and sometimes vouches for Spenser at crime scenes where Spenser shoots someone.