Here's the easiest way to comprehend the longevity of Scooby-Doo: Casey Kasem has been doing the voice of Shaggy (Norville Rogers, if you insist on his given name) for longer than he hosted his weekly Top 40 radio show.
Advocate definition, to speak or write in favor of; support or urge by argument; recommend publicly: He advocated higher salaries for teachers. See more. Dictionary.com
Exponent can also refer to someone who is an advocate, but it tends to refer especially to someone who stands out as a shining representative of something, and in addition it has retained its earlier meaning of one who expounds.