The Observer played by, Michael Cerveris

(fringepedia.net)

Who else but Michael_Cerveris could play the roll of The Observer? Of course there is more than one Observer in the Fringe TV series, but MC seems to be shown in every episode.

http://fringe-theobserver.blogspot.com/

Fringe_(TV_series)#Reception

http://fringeseason4.com/

The Observer is the quintessential character to explore or edit key moments in time. The Observer seems to be MIB Light.

http://jfvrinfo.blogspot.ca/search?q=MIB