Nelson needs Lisa to keep him off the streets, and help him gain a decent education. She's also the one female who cares about him. Not even his own mother gives him enough love.
Lisa needs Nelson to stand up against the many fickle children at Springfield Elementary who put her down, along with the Caitlins of Waverly Hills and many educational and government figures, who belittle her views. She needs his man-power to get the job done, whenever civil reasoning fails.