Melvin Sattem is a supporting antagonist in the 4th and final season of Ozark. He is a private investigator, a former homicide detective in Chicago, and a recovering drug addict and alcoholic.
Mel is a Chicago based private investigator. At some point before season 4, Mel worked for the Chicago Police Department as a homicide detective. Mel was an over achiever at work, suffering from insomnia, and addicted to drugs. He was either terminated, or he resigned from the police force when another officer caught him stealing and snorting cocaine from an evidence locker.
Following the death of Helen Pierce, Mel is hired by Helen's ex-husband Gene Pierce to investigate her disappearance, and get her final divorce signature so Gene can marry his fiancé. While searching for Helen, Mel comes across the Byrde family and begins to suspect their behavior and motives surrounding Helen's disappearance. Helen's ex-husband drops the case when Mel can't find her, at the insistence of his daughter (who knows she's dead).
Mel later gets hired by Nathan Davis, who wants help finding his missing son Ben Davis. Mel eventually concludes that Ben disappeared at the hands of the Navarro cartel. He suspects Wendy and Marty's involvement but can't pin anything straight on them. He agrees to testify at a court hearing where Nathan fights to take custody of the kids, but he doesn't show because he accepted a job offer as a police man (which was arranged by the Byrdes pulling strings). He goes to Chicago to work but his conscience brings him back to the Ozarks.
Eventually- he figures out that Ben's ashes are hidden in a cookie jar. He finds them in the Byrdes' house and waits to confront them. He tells them he caught them until Jonah points a rifle at him and shoots Mel dead.
Mel is cocky, curious and persistent, often annoying the people whom he is investigating by showing up unannounced, asking direct personal or incriminating questions, and pestering law enforcement when they refuse to assist him. Oddly enough, this does not affect his ability to ask unsuspecting strangers for information. Despite his lack of boundaries, Mel is cool tempered and objective in his work, not threatening any of his suspects with consequences from himself or law enforcement and not caring much about what other crimes he unearths so long as he closes his case.
According to Jim Rattelsdorf's research, Mel was quite competent as a homicide detective.