Zelig

Etymology
From the surname of the character Leonard Zelig from the 1983 film  who meets many famous people in the 1920s and 1930s and, apparently out of his desire to fit in and be liked, unwittingly takes on the characteristics of the strong personalities around him.

Noun

 * 1) An ordinary person who can change their appearance or behaviour to suit their surroundings, especially one who is unexpectedly associated with important events or people.
 * 2) An unimportant person who appears at multiple important events.
 * 1) An unimportant person who appears at multiple important events.