Russell is a pharmacist and was a friend-of-sorts of Charlie. He has been known to both use and sell illegal drugs.
Russell manages and owns a pharmacy that Charlie goes to on occasion. Russell also has three ex-wives, all of which he pays alimony to, which may or may have not been the cause of his constant abuse of medications in his pharmacy. It mentioned he has children with his ex-wives because he tells Charlie he has trouble paying at least one penny in child support.
The first time Charlie was seen with Russell was when he was asking about the effectiveness of condoms in preventing pregnancy. Another time was when Chelsea was sick with the flu and having her period, and Charlie needed to go to the pharmacy to get tampons and flu medicine. Russell gave Charlie a special cold medicine with codeine, but when Charlie asked about codeine requiring perscriptions, Russell simply replied, "What are you, a cop?", something Russell often says when Charlie questions his business ethics.
One time when Charlie was sick and Chelsea was going to the pharmacy, Charlie told her, "If you get help from Russell, count the pills." Stating quite blatantly that Charlie lacks trust in Russell. The one time Charlie was at Russell's pharmacy for sleeping pills, but Russell convinced him to buy medicated marijuana instead, which caused Charlie to have hallucinations of several of his ex-girlfriends complaining about him.
Charlie once tried to set Russell up on a date with Evelyn on her birthday, but showed up at her birthday party with a thirty-year old woman in Season 8. Evelyn eventually leaves the party with Russell's date for a lesbian one night stand. Russell told Alan and Charlie at the party he wants to open a "prehab", a place celebrities to stay for one month to do drugs.
Russell attends Charlie's funeral in the Season 9 premiere episode. At the service, Russell demands $38,000 for drugs that Charlie owed him, but Alan says it's something he has to discuss with the lawyers, but he says he doesn't want to involve the law. He has not been seen or mentioned since this particular episode, and it is likely that he may never appear again as Charlie has died. But he could appear again in the near future. He reappeared in "Welcome to Alancrest" during Season 10 when Alan took Walden to see him.