Lester Diamond (portrayed by James Woods) is a minor antagonist in Casino (1995).
Diamond is first introduced while Ace is narrating his first encounters with Ginger McKenna; Diamond is described as the 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 Ginger probably feels very sorry for him. Soon after Ace and Ginger get married, Ginger receives a phone call from Diamond where he states that he misses her and that she must miss him as well. Ace knows that Ginger is speaking with Diamond, 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 money was meant for Diamond, and Ace finds Ginger and Diamond sitting together at a diner. Here Ace tells Diamond that he better stay away from Ginger, and as Diamond leaves the diner a few of Ace's men follow him to his car and beat him of (Ginger screams at Ace in anger at the site of Diamond being beaten up, and Ginger states that the situation was her fault, thus Diamond 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 Diamond. Thanks in part to Diamond's rude and abusive behavior, Ace manages to convince Ginger to come back to him. Diamond is not seen for the rest of the movie.