Miranda Bailey, played by Chandra Wilson has worked her up the ladder as well, starting as a resident and now acting as Chief of Surgery and finally Dr. Richard Webber, played by James Pickens Jr., remains from the inaugural season as well.
Alex Karev, played by Justin Chambers, started as a moody intern, and is now a senior member of the surgical team.
Incredibly, it is worth noting that the show of the nine original main cast members, four are still with the show and have appeared in every season, including the star, Ellen Pompeo, who of course plays Meredith Grey.