Dr. John Carter.

We first meet Carter in the pilot episode, he is Benton's Third Year Medical student, however he has absolutely no clinical experience!!

Carter learns quickly and is soon competent at a variety of situations, eventually earning time in the OR.

Carter comes from a rich family which Benton resents, Benton still has debts from medical school.

Susan has a strong friendship with the young John Carter.

Has a brief relationship with the promiscuous Liz gives Carter an embarrassing infection (Which Jerry kindly posts the test results on the notice board !!).

Carter practices his suturing often on pig's feet.
Carter gets the Surgical Sub I after another student accepts the Orthopedic Sub I, this prompts Carter to rewrite his evaluation of Dr. Benton (which he has to get back from Dr Hicks).

Carter arrives back late from his summer off.  He returns as a fourth-year medical student doing a surgical sub-internship (sub-I).  Carter is a very caring person, who gets on well with the patients and seems to really care.  A classic example of this is at the end of Season 2 when Carter misses his graduation because he is spending time with a girl awaiting a liver transplant.
Carter and Harper Tracey go out, an old friend of Harper's, Dale Edson, forces Carter to compete, eventually making a fool of himself and causing Harper to break up with him in Episode 044 " Fire In The Belly".

Carter becomes a surgical Intern and is very good at it, but he decides it is not the career he wants.

Season 3 sees the arrival of Abby Keaton a renown Pediatric Surgeon, as Benton's student he gets to known Abby rather too well. He then has to hide his relationship from Benton.

 Carter has a change of career from surgical intern to emergency medicine intern, meaning that Carter repeats his year as an Intern (without pay).

By this time Carter is a superb Doctor and leader' which was demonstrated when he was arrested for doing something good for a patient.  It is also demonstrated when the ER becomes contaminated with Benzene, Carter takes control, evacuating the ER.

During Season 4 Carter becomes very friendly with new Pediatrician Anna Del Amico, when Carol eventually persuades him to ask Anna out Anna says no, but there is still something special there.
Then when Anna's ex-boyfriend and ex drug addict Max turns up near the end of Season 4 Carter is less than nice to him, do we detect jealousy ?

Anna has left (gone back to Philadelphia for an ER Pediatrics job and to be with her family and Max). Carter has his own Med Student Lucy Knight.

Carter is getting paid this year, he has also moved into halls of residence as a warden.

After a party in the halls of residence gets out of control, with two students taking drugs, Carter loses his job and has to move out. Carter rents an apartment from Kerry and also has a new girlfriend, Roxanne.

Lucy kicks Carter in the chest whilst practicing kick boxing cutting his head, whilst cleaning and stitching up the cut Lucy and Carter start kissing and fooling around. Carter points out that they shouldn't be doing this and that he could get in trouble. Chuny discovers them, Carter has a larger bruise on his neck and the whole ER staff are making fun of him.

Dr. Carter is now a Senior Resident.  In Episode 126 "Be Still My Heart", Lucy and Carter are attacked by a Schizophrenic, both are stabbed, Carter is taken to the OR by Benton and pulls through. Lucy has a rougher ride requiring defibrillation and surgery before throwing of a Pulmonary Embolism, which she dies from.  Carter returns to work in the ER in Episode 129 "Under Control".

In Episode 135 "May Day", we learn that Carter is addicted to pain killers that he has been stealing out of the exam rooms to cope with what happened to him and Lucy.  He leaves with Bento to Atlanta for drug rehab after being given an ultimatum of going or being fired.

Carter flies back to Chicago from rehab in Atlanta in Episode 136 "Homecoming".

We finally get a glimpse of Carter's mom and dad in the season opener, "Four Corners".  They are back in the states to attend Carter's grandfather's funeral.

Carter's 'Gamma' isn't doing well, so he takes extra good care of her.  Carter's mom and dad are not doing so well, so they split up.  This puts Carter in the middle.  Eleanor takes care of an orphan boy named Mickey who has Leukemia, the same disease that killed her other son.  Carter does not like her re-living the past through him.  Mark Greene has a heart-to-heart talk with Carter on his last day in the ER.  He tells Carter that he has been there the longest and that people will be looking to him for direction.  He pretty much tells Carter that he is going to be the "Top Dog" in the ER after he has gone.  Carter reads the letter that Dr. Greene sent to the ER while he was in Hawaii during his last days.  He is very upset about the news of his mentor's death.  Carter and Abby get a little closer during the Lockdown of the ER when it is suspected that they may have been exposed to a small pox breakout. Something we all know (from the spoliers *fingers crowesed*) will end up with a loving relationship.