Sierra McCool is a character and antagonist in The Replacements.
She is the popular girl at school, and is Riley's arch nemesis. She is constantly competing with Riley for the affections of Johnny Hitswell. In the second season, when Riley and Johnny become a couple, her crush on him persists, and her aim becomes to break them up so she can be Johnny's girlfriend. She has her own clique (Jennifer and Claudia) and a huge, conceited ego. She is seen to have another side in which, she is infatuated with Star Journeys, a Star Trek parody which Todd also likes. This forges a bond between the two, which doesn't last long when she becomes a cheerleader again (though they both promise to always be space pals). It is mentioned in the series finale that she gets her own Fleemtel Phone. She has fair skin, freckles on her cheeks, long black hair in pigtails, and green eyes.
In seasons 1-2, she wore a long-sleeved red dress with maroon patterned stripes on it, and white boots.