A brand agent represents a brand similar to how a celebrity agent represents a movie star or a sports agent represents a football player. Their role is to create the brand persona, give it a voice, an image and then manage the image, the presence, and ultimately where and when the brand resides, how its used and where/how it earns money.
Brand agents are the brains behind a brand ensuring that they are where they should be in terms of sponsorships if celebrity talent is supporting or representing the interests of the brand. A brand agent is also the brand strategist and the voice of the brand from behind the scenes. The brand agent decides who gets to use the brand, where and how and negotiates the fees.