Lovejoy. Was it his first name? Or his last? Deliberately left ambiguous.
As an homage to the original Battlestar Galactica (where nobody had surnames) several characters in the revival series were only given one name (Gaeta, Tyrol, Cally, Cottle, etc). We later found out their first names (in Cally’s case her lastname). All except for Dr Cottle.
Bewitched - We never learn Samantha’s maiden name despite a parade of her relatives appearing on the show .
We never learned *any *of Agent 99’s names. Or the Chief’s. And Larabee - first or last?
There were two young aides who always appeared together on The West Wing. Even those senior staffers who could remember their names never seemed to be able to tell them apart, even though one was Anglo and the other was Asian.
They go one better with his sister, who is just ‘Kyon’s Little Sister’.
For that matter, several of the pilots are known primarily or entirely by their call-signs (I don’t think we ever learn Racetrack, Skulls, or Flattop’s names). They even lampshaded it once: Adama addresses a pilot by name, and she’s surprised, not that he remembered her name, but that he ever knew it.
I thought Starbuck was his call sign.
ETA: Er, his or her. Or what.
In the original series, Starbuck appeared to have been his real (and only) name.
In the new series, Starbuck was, indeed, her call sign (real name was Kara Thrace).
Some would say twice:
99 used the name “Susan Hilton” in one episode, but later told Max that WASN’T her real name.
The Chief’s full name was never revealed, but a few times, he was given the first name Thaddeus.
This is the case with all the original series pilots - their name from the original (eg: Apollo, Boomer, Starbuck, etc), becomes their call sign, and they’ve got a real name aside from that (Lee Adama, Sharon Valerii, Kara Thrace).
The civilians and officers whose names carry over (eg: Adama, Baltar, Tigh), their TOS names become their last names, and they gain first names (William, Gaius, Saul).
(Of course, several of those names are altered by spoilers.)
Disco Stu
I had forgotten about that. When Endorra was asked her last name she got around it by saying, “You’d never be able to pronounce it.” 
Spock said the same thing to the blonde in the planet-of-the-happy-spores Star Trek episode, come to think of it. His mom Amanda said something similar in “Journey to Babel.”
Just watching a repeat of “The Andy Griffith Show” and Sara the telephone operator was a reoccurring mystery character who was never seen or heard.
Juanita, the diner waitress, was serenaded by Barney several times but was never seen or heard.
Long time girlfriend of Barney, Thelma Lou’s last name was never revealed.
If I recall correctly, in one episode it was Spock’s frist name that couldn’t be pronounced, and in the other episode it was his last name.
Which means that “Spock” is either his middle name, his nickname, his pseudonym, or his “Name to be Used Among the Barbarians.”
MacGyver never had his first name directly revealed. A lot of people just called him Mac.
Though near the end of the series, he meets an adult son he never knew about. The boy is called Sam. Eventually it is revealed to actually be his initials. His actual name is Sean Angus Malloy. He says his mom told him his middle name was his father’s first name.
No, a few episodes before that MacGyver dreams (is sent back?) to King Arthur’s time and his name (I guess it was also his ancestor’s name? Can’t remember exactly) was written on a wall. The woman he was with says “OOh, nice name” and he says “Well, maybe around here, but where I come from…”
Spock’s first name is Mister.
Duh.