A helper was an individual who aided another in performing a task.
In 2368, Clara Sutter was Keiko O'Brien's helper in USS Enterprise-D's arboretum, planting and watering nasturtium seeds. She was explaining how to do it to her imaginary friend "Isabella", when she was interrupted by an unknown lifeform that pretended to be Isabella. "Isabella" then suggested they finish planting later and explore the ship instead. (TNG: "Imaginary Friend")