in the series, the elementalists must battle an evil, and when one element is absent, they usually cannot carry out their objective. they are strongest when united.
also, each character's personality is usually reflected on their element. ex: fire is rash and impulsive; water is calm and collective; earth is nurturing and loyal; air is inquisitive and curious
Avatar: The Last Airbender
of course, avatar and w.i.t.c.h are the best. :)