Colleen was the eldest of the three Cooper children (Matthew, Colleen, Brian) adopted by Dr. Michaela "Mike" Quinn and Byron Sully. She was initially portrayed as a headstrong, smart, and sometimes boy-crazy teenager who matured significantly during her time in Colorado Springs. Key storylines for Colleen included her romance with Andrew Cook (played by Brandon Douglas), her desire to become a teacher, and her early struggles with her new family.