Poster child

The term poster child (sometimes poster boy or poster girl) originally referred to a child afflicted by some disease or deformity whose picture is used on posters or other media as part of a campaign to raise money or enlist volunteers for a cause or organization. Such campaigns may be part of an annual effort or event, and may include the name and age of a specific child along with other personally identifiable attributes.

The definition of "poster child" has since been expanded to a person of any age whose attributes or behaviour are emblematic of a known cause, movement, circumstance or ideal. Under this usage, the person in question is labeled as an or archetype. This signifies that the very identity of the subject is synonymous with the associated ideal; or otherwise representative of its most favorable or least favorable aspects.

...
Wikipedia