One might question the extent to which Dr. Jekyll and Mr. Hyde are in fact a single character.
Until the end of the novel, the two personas seem nothing alike—the well-liked, respectable doctor and the hideous, depraved Hyde are almost opposite in type and personality.
Stevenson uses this marked contrast to make his point: every human being contains opposite forces within him or her, an alter ego that hides behind one's polite facade.
Correspondingly, to understand fully the significance of either Jekyll or Hyde, we must ultimately consider the two as constituting one single character.