You may not recognize Alejandra Gaitan’s face, but her voice and especially a certain area of her body have won her millions of views (and a number of sponsorships) on YouTube. Gaitan, who calls herself “The Reply Girl,” has carved out a career making YouTube videos commenting on other videos on the site. While a major source of YouTube’s democratic appeal is that everyone from your grandmother to a preteen can upload and discuss videos, Gaitan’s unconventional methods have earned her both advertising...