Brand ambassador

Brand ambassador (celebrity spokesman) is a marketing term for a person employed by an organization or company to promote its products or services within the activity known as branding. 

Rogger Federer, Mercedes Benz Brands ambassador ( blog.mercedestucson.com )

The brand ambassador is meant to embody the corporate identity in appearance, demeanor, values and ethics. The key element of brand ambassadors lies in their ability to use promotional strategies that will strengthen the customer-product/service relationship and influence a large audience to buy and consume more. 

Yara, arab singer, Piaget brand ambassador ( piaget.com ) 

Predominantly, a brand ambassador is known as a positive spokesperson appointed as an internal or external agent to boost product/service sales and create brand awareness. Today, brand ambassador as a term has expanded beyond celebrity branding to self branding or personal brand management. Professional figures such as good-will and non-profit ambassadors, promotional models, testimonials and brand advocates have formed as an extension of the same concept, taking into account the requirements of every company.
source : wikipedia.com