Mia, played by Emmanuelle Vaugier, is Charlie's ex-fiancé.
On the Show
Mia met Charlie at a cafe while she was reading a dance magazine. Charlie was out with Alan and he tried "talking Mia up". Mia showed that she wasn't interested in him, but Charlie announced that he had a nephew Jake who really enjoyed dancing. Jake took the lessons, and had a crush on his new teacher. Mia gave Charlie a chance with her, but Jake was very envious of Charlie. As Charlie enters his first serious relationship, he finds it difficult to go without having sex, and asks Mia if he can have sex with other people. They get into a huge fight, but make-up. Later when they do finally consummate the relationship, Charlie is so nervous he kept screwing it up. Eventually, new problems rise as Mia tries to makes Charlie take up a healthier lifestyle, giving up cigars, beer, meat, and making him take jogs with her. Charlie kept doing all these things in secret, and when Mia discovered them, they broke up. Mia left Malibu, but eventually came back with her dancing troupe, and asked Charlie to donate sperm to her so she can have a child. Charlie instead asks her to marry him, which she accepts. Later on in the series Charlie and Mia go to Las Vegas - with Alan and Kandi - to get married. Right before they get married, Mia asks when Alan is moving out of the house. Charlie refuses to kick out Alan for Mia, so they split-up. Alan and Kandi then get married using Charlie's ring he had for Mia.
In season 5 episode "Rough Night in Hump Junction", Charlie realizes that he stills misses Mia and tries to get her back despite she being about to marry, and he is greeted by Mia's dad Sam with a punch. Later is revealed that her marriage didn't work out and she moved back to Malibu.
In season 6 she returns.
In the first episode of Season 7, she wanted Charlie's musical help for her new career as a singer. Charlie at first thought it was a bad idea due to him now being with Chelsea, but Chelsea suggested he help her. She was a terrible singer, but for some reason thought she was really good. She revealed she wanted to get back together with Charlie, but he chose to be with Chelsea. When Charlie took marijuana and envisioned several ex-girlfriends, Mia was among them.
In the Season 9 premiere, "Nice to Meet You, Walden Schmidt", Mia was present at Charlie's funeral. Here, she wanted the casket open so she could see his body.
Trivia
In the Season 3 finale, "That Pistol-Packin' Hermaphrodite" both Charlie's and Mia's families meet in a meal that turns out to be a disaster; Charlie describes Mia's father as being cheap (just like Alan) However, in Season 5 episode "Rough Night in Hump Junction" Mia introduces Charlie to her dad like if it's the first time they met.