For a given brand, a villain is another, competing/alternative brand that presents a series of problems – not just for the given brand’s own business, but also for the given brand’s end users or customers.
For example, for 1990s Apple, a competing brand, Windows, presented a series problems to both Apple and to a segment of Apple’s users and customers. Apple named Microsoft/Windows as its villain publicly in various advertising campaigns.
Sometimes a brand doesn’t name its villain or does so obliquely, as in the example the Roam notetaking app calling out Evernote as its villain by using the medium of “testimonial”.