The Smurfette Principle was first raised in 1991 by Katha Pollitt in NY Times, speaking of the tendency of mass media to include only just one female character amongst their leads, citing The Smurfs as example. Interestingly this tendency is little but subdued in modern media today. The following video by Anita Sarkeesian takes us through more.  

Truth be told, I am ashamed I never noticed the trend so starkly until raised by the video. Now here's to urging us to open our eyes to more.