W.I.T.C.H. is a French animated series based on an Italian comic book of the same name.
"Quintessence," the word often spoken by Nerissa, is the mythical fifth element, believed by ancient scholars to be the material of celestial bodies.
In the Italian comics, Hay Lin's grandmother, who gave the girls the heart, dies within the first issue. However, in the show, she is kept alive.
Will's abilities and the origin of the girls' powers are never explained until the middle of the second season.