This article is from the ER FAQ, by Rose Cooper firstname.lastname@example.org with numerous contributions by others.
At the end of the second season ("John Carter, M.D.",
), Susan Lewis was one of the candidates for the position
of ER`s chief resident, a title which was then currently held
by Kerry Weaver, and the year before by Mark Greene. Mark
made a backchannel deal with Kerry to get Kerry to support
Susan`s candidacy for the position, but Susan withdrew her
name from the running. The position then went to someone named
Dr. Linda Martins.
Dr. Martins has never been seen on the show, and her name
has not been mentioned since that episode, which puzzles fans of
the show who tend to keep up with stuff like this. The simple
answer is that now that the main characters are advancing, none
of them are qualified to be chief resident of the ER anymore,
and so the writers have simply elected to ignore the position
for dramatic reasons. Mark and Kerry are now both attending
physicians who work in the ER, so they pretty much function in
the same capacity.
(Now that he`s switched his residency from surgery to
ER medicine, it`s possible that Carter will be qualified for this
position in two or three years.)