A native of El Salvador, Rosario was a schoolteacher who received a bachelor's degree in clinical psychology from the University of Texas, and almost got a Masters Degree (three credits short); she, at some point, also attended Business school. She also fought in the resistance and was a sniper. At some point, she moved to the Bronx - assumedly while studying Business. While there, she also performed with a then-unknown Jennifer Lopez, including a production of Tea for Two at the Shalom Retirement Center. At one point, she was also paid to tape pornographic videos of Tommy Lee.

One of Rosario's brothers is a political prisoner, and remarks made by Karen suggests she has children, but they are possibly still in El Salvador.

In 1985, she met Karen Walker at a club, while working as a cigarette lady, and went to work for her as a maid, which was where she was working when the series began in 1998. Rosario and Karen are incredibly close, but they constantly bicker and try to outdo each other in snide comments, causing strangers to think they are mortal enemies.

Although she had been mentioned by Karen since the pilot episode, Rosario's first appearance was in the season-one finale, "Object Of My Rejection." To keep her in the country, Karen had her marry Jack McFarland, her gay friend, to get her green card. The marriage was later terminated at the end of season two, when Rosario asked for a divorce to pursue other romantic interests.

Although she briefly "cheated" on Karen by also working for Karen's rival Beverley Leslie, and once had to take the rap when Karen attempted to smuggle pearls in to the country from the Virgin Islands, Rosario has remained faithful to her mistress. After Karen's separation from her husband Stan, her money was frozen and Rosario slept by her side in the limo for several days. After Stan's apparent death, while throwing his ashes to sea, Karen was pushed overboard by Stan's mistress Lorraine, and Rosario jumped into the ocean to save her. They were picked up by a Russian ship and later stole a jet ski to make it back to their boat. Stan left $20 million dollars to Rosario, however, she was not elgible to get it until 20 more years of service to Karen.

Rosario was one of few people who Stan remained in contact with after he faked his death (in season five). When Karen learned that Rosario had kept this news from her (in season eight), she fired her. Rosario worked as a bar attendant until Karen ultimately took her back.

In the show's series finale, Rosario was shown to have lived with Karen and Jack in the years after the show ended, growing older with them and possibly still working for them despite being in a wheelchair (though she states that she is only in the wheelchair due to a sprained ankle).

Rosario typically is seen wearing a Members Only jacket on top of her maid's uniform. She has a particular fondness for sandwiches from Subway.

Morrison was a guest star in the first two seasons, but from season three until the show's end she was listed as a main cast member in the episodes she appeared in.