It can be hard to write critically about “Curb Your Enthusiasm.” For one thing, after 20 years the show is a bit critic-proof. The audience for “Curb”  is locked in at this point; one is on board for the periodic ten-episode journey through well-meaning gripes and grudges, or one is avowedly not. What’s more, the show’s best and worst qualities tend to be unusually entrenched with one another, such that an episode can contain bits that work beautifully and some that fall utterly flat. Were one designing the show now, it’s impossible to imagine coming up with latter-day “Curb” from scratch; its gnarls and knots have accreted over time to build a show that appears entirely congruent with Larry David’s worldview.

And while the show has always been nimble in coming up with new turns of phrase or fresh terms along which to hold an argument, finding entirely new comic creations has been slightly rarer. That’s why this current season, the show’s eleventh, feel like a watershed: It brought us Irma Kostroski.

Perhaps not since the introduction of J.B. Smoove’s Leon Black has “Curb” witnessed such an electrifying character addition as the Santa Monica city councilwoman. On first meeting, Irma and Larry instantly loathe one another — a curmudgeon-to-curmudgeon connection that suggests big things ahead. (What is more “Curb” than feeling a strange sort of respect for the people one loathes?) And once Larry realizes he can leverage his acquaintance with Irma into getting himself out of a jam, he puts on a show of Davidian charm. Among Irma’s astonishing qualities as a character is what she brings out of the show’s lead; David, running an multiple-episode double game of dating a woman he cannot abide in order to attain a goal, is doing some of his best comic acting in memory.

But let’s not take anything away from the performer who brings Irma to life. Until the credits rolled on her first episode, I assumed Irma was played by some character actress I’d never before seen. Instead, it’s Tracey Ullman under the red hair and statement jewelry. Ullman is unafraid to play big, broad notes — a sort of grotesque physicality, as when Irma describes her own gas — but she doesn’t play the part with loathing, either. The challenging and fearless element of Ullman’s performance is that Irma is a real person, more so than most of Larry’s entertainment industry peers. Proudly dowdy, she’s plainly unmotivated by the material. Rather than keep up in the thrust and parry of conversation, she misreads Larry’s intentions at every turn, and comes up with turns of phrase that are almost poignantly uncool.

And on a show which proudly displays its allergy to emotion, now comes a character foolish enough to believe that she’s found real love in Larry. (The naïveté, or perversity, of a character seeing a love-object in the ultimate loner is among the key jokes at play here.) There’s a yearning underneath the oddity that only amplifies Larry’s conundrum. One senses that he won’t get away from this woman without ending up in a worse predicament than the one in which he began. (Perhaps we’ll find out when the current season ends Dec. 26.) For now, his torment is private, as he has shown a wonkish, idiosyncratic woman the time of her life, and allowed her to release far more of her inhibitions than he’d ever have wanted to see.

But just like “Curb” viewers, Larry may yet find something to love — or at least respect deeply — in the woman with whom he cannot stand to share a bed. That tricky, perfectly disastrous alchemy is at play in certain scenes in the Irma arc, as Larry, decrying his poor judgment in getting involved with her, dances on a knife’s edge of contempt for middle-aged women in general. But Irma is such a specifically drawn character, with a need for Larry so perfectly calibrated both to trigger his narcissism and to repel him, that Ullman’s work — and the show — have so far landed on the right side of the line. And this bold, strange, bawdy performance has given new life to the most indefatigable of TV comedies.