Richard Speight’s semi-recurring Trickster character on Supernatural.
That was Allan Arbus. He was married to photographer-of-odd-things Diane Arbus.
I like Jimmy Palmer a lot, too.
Loved Frasier’s agent Bebe Glazier. [This IMDB photo does NOT do her justice. In the Bebe role she was gorgeous.] I loved the episodes that featured her. I’ve saved on my DVR the one where Frasier is supposed to get her to quit smoking so she can marry the octogenarian Texas oil man, Big Daddy. Alas, he dies on the way down the aisle. Also the one where she stages a fake suicide attempt so Frasier can “save” her on the evening news. When they’re on the window ledge, she leers at the news helicopter, “Come to Mama!” At the end Frasier asks her, “Do you have anything to say for yourself???” She replies with a sly smile, “Aren’t you glad I’m on *your *side?”
Ditto on Kimball Cho, but now that he’s with the FBI, he’s not wearing those tight shirts anymore. 
I wish they hadn’t gotten rid of Gerald Jackson (played by Pancho Demmings), Ducky’s original assistant on NCIS, so quickly. He could have been a very interesting character, but he was never really developed before they quietly wrote him out of the show.
Did Ari shoot him?
Omar on The Wire.
In the shoulder, yes. He made a couple of appearances after that, but he never did return to his job.
Creed Bratton from The Office (US version)
Oh yeah… it wouldn’t be an SDMB poll thread without most people getting the question wrong. Interestingly enough, the OP got it wrong too.
If you appear in every episode, you are a “main character.” You might not be the “star,” but you’re a “main character.”
Thanks for the nitpick. I did try to give examples of which members of the cast I was after. But it’s fine to point out the lack of precision in the wording.
Feel better?
I about died laughing when Niles said that perhaps she “…wanted to mate with a human male to bring about the Apocalypse.”
Nope, still pretty confused. All the words you use in your OP point to “recurring characters,” characters that didn’t appear in every episode, like Newman or Uncle Leo from Seinfeld, but were still memorable. But then your example list is a bunch of characters who are actually “main characters” that appear in every episode but aren’t considered a “star” from a marketing perspective.
So which are you looking for, “recurring characters” or “lower tier main characters”?
I’ve always thought a “supporting character” or “supporting actor” was there to help the star(s) shine, regardless of how many episodes he or she was in. 
Even James MacArthur was never officially anything other than a “featured player” on the old Hawaii Five-0. Jack Lord pitched a bitch when TV Guide referred to Danno as his co-star. The same was true for James Doohan et al. on ST: TOS. Shatner would never have stood for it.
Eddie Haskell - Leave it to Beaver
Floyd Lawson - The Andy Griffith Show
Mr. Carlin - The Bob Newhart Show
Tell you what: why don’t you rewrite the OP with your choice of words and see how much clearer you can be. I sense that most posts got the gist of the OP and answered with what I was hoping to see.
I enthusiastically second Mycroft from Sherlock, as well as Newman from Seinfeld.
Also, although it’s not a TV show, I like to think that if Shakespeare lived in 2014 he’d write soap operas, so I have to mention Benvolio and Mercutio from Romeo and Juliet. The play is (IMO, of course) one of his worst, from the overly soppy, melodramatic plot to the incredibly irritating main characters of, well, Romeo and Juliet, but those two characters somehow saved it. I would happily read several novels filled with half-comical, half-philosophical, pun filled musings between Benvolio and Mercutio.
All of Al Bundys pals on Married With Children
Jefferson
Griff
Bob Rooney
Ike
Officer Dan
I still don’t know which characters you’re looking for, “recurring characters” or “lower tier main characters”?
I really don’t have a problem with your subdividing your reply into as many non-main-character subgroups as you’d like, if it makes it easier to name the ones (not the main stars) you count on for making the show work.
And after that ill-fated night she and Frasier spent together, when Niles came over and saw what had happened, he said, “I would have thought she killed after mating.”
