cast evolution

This is the cast from the first season.
L to R, back row: Sawyer, Sayid, Kate, Jack, Charlie, Boone, Hurley, Shannon, John.
L to R, front row: Jin, Sun, Michael and Walt, Claire.



Season 4 promotional photograph.
L to R: Hurley, Claire, Sawyer, Kate, Jack, Jin, Sun, Juliet, John, Sayid, Ben, Desmond.



Finally, one of very few cast promotional photos from season 6.
L to R: Ilana, Richard, Claire, Sayid, Kate, Sawyer, John, Jack, Jin, Sun, Miles, Ben, Hurley, Frank.

Obviously, there are many characters that can be considered "main" characters, as they carry over from season to season. Sometimes, minor characters from previous seasons become main characters, for example Richard in the final season. He has been a character on the show for several years already, and his promotion to main character status is pretty exciting for me, I dunno about everyone else.
Also, I wish these photos could be bigger on this blog, but alas.