Gus Rigsby was Harry Morgan's oldest friend on the force before he was murdered. He had been Harry's best man, and vice-versa. Harry was Betsy Rigsby's godfather, Gus' daughter. Otto Valdez murdered him, after Otto got out of prison, when Gus and Harry had been on the force around 20 years. Gus did not serve in the Vietnam War, unlike Harry. He was Uncle Gus to Dexter Morgan and Deborah Morgan. Gus appears in Dexter by Design.
Before he was murdered, he had asked Harry to help him kill Otto. Harry refused, saying that in 'Nam he learned he wasn't a natural killer'. Shortly afterwards he died and thereafter Harry started training Dexter to become the Dark Avenger serving Dark Justice, when the Light side of Justice failed.