There's one character not named above who first appeared in Season 4 Episode 1, and who has been in 41 episodes of the series so far (and has been portrayed by a total of 4 actresses over the years) - Judith.
Not a speaking role for a few seasons, but always integral to the plans of Rick and several of the others.
Shocking there aren't a few more who've made it this far. They've killed so many major characters.
Also, Aaron, from Season 5 Episode 10.
Are there others?