Lester Diamond (portrayed by James Woods) is the tertiary antagonist of Casino (1995).
Diamond is first introduced while Ace is narrating his first encounters with Ginger McKenna; Diamond is described as a low-life former boyfriend of Ginger who abuses drugs (including cocaine). Ace presumes that Ginger only affiliates with Diamond because they have know each other for a long time and that Ginger probably feels very sorry for Diamond. Soon after Ace and Ginger get married, Ginger receives a phone call from Lester, where Lester states that he misses her and that she must miss him as well. Ace knows that Ginger is speaking with Lester, but he trusts her enough at that point to not make a big deal out of it.
Later in the film Ginger asks Ace for $25,000, but refuses to explain what the money is for. Ace figures out that the movie was given to Lester, and Ace finds Ginger and Lester sitting together at a diner. Here Ace tells Lester that he better stay away from Ginger, and as Lester leaves the diner a few of Ace's men follow Lester to his car and beat him of (Ginger screams at Ace in anger at the site of Lester being beaten up, and Ginger states that the situation was her fault, Lester did not need to be beaten up).
When Ace and Ginger file for divorce, Ginger takes their daughter, Amy, to Los Angeles where they plan on flying to Europe with Lester. Thanks in part to Lester's rude and abusive behavior, Ace manages to convince Ginger to come back to him. Lester is not seen for the rest of the movie.