WHEN ANDY RICHTER takes his last bow as sidekick-in-residence on Late Night with Conan O'Brien tonight, after a seven-year run with the program, he will be leaving behind more than just a critically and commercially successful late-night franchise. He'll also be leaving a spot on the talk-show couch that he single-handedly reinvented -- and one that will not be easily filled by the likes of Max Weinberg. Reviled during the show's turbulent first years -- Time magazine notoriously called him a "superfluous appendage" -- Richter quietly turned himself into the show's most interesting, and multifaceted talent. A brilliant remote correspondent, Richter perfected a deadpan, furrowed delivery that played exquisitely against Conan's manic presence in the studio. At its best, it threatened to make Conan the superfluous appendage.

Until Richter came along, the joke was that moving from a talk-show sidekick to the Publishers Clearing House spokesperson counted as a promotion. It was, to say the least, not one of network television's more illustrious perches: McMahon's you-are-correct-sir pandering slid effortlessly into Paul Shaffer's cloying meta-showbiz act, which probably would have slid into Branford Marsalis's act if you could remember what the act was. (All of which coalesced in Hank "Hey Now" Kingsley's needy, airhead role on Larry Sanders.) You could fairly say that Richter wrestled with the demons of McMahon's precedent for his duration on Late Night, and that wrestling generated some of the darkest and best material on the show. In one segment from the mid-nineties, Richter goes to the MTV Music Video Awards with a reel of hideously remixed music videos featuring him lip-synching to the hits of the day. Richter shows his "reinterpretations" to a motley assortment of celebs, asking for advice about breaking into the music business. Halfway through the segment, B-52s frontman Fred Schneider listens to Richter's story, and then consoles him, "The talk-show business isn't treating you so well." Richter just nods his head glumly.

Richter's battle with the ghosts of sidekicks past had its own precedent in the talk-show pantheon, but its roots were behind the desk, not beside it. Originally hired as a writer for the show, after a brief and nondescript career capped by a role in the Chris Elliott wannabe-cult-classic Cabin Boy, Richter found a place on the couch after two weeks on the team -- echoing Conan's own unlikely migration from the writer's room to the main stage. But while Conan always had the giddy demeanor of a longtime writer who'd suddenly found himself in front of the camera, Richter created a more complicated mix: a comedy writer's knack for generating material on the fly, tempered by a sense of unease with himself, and the second-banana role he'd locked himself into. Richter's digs at himself made him the true heir to Letterman's "I hate myself" phase from the late eighties. His normal mode was a Sahara-dry delivery, with a strong undertone of self-loathing. At one point on Thursday night's show, Conan made an offhand reference to it being Andy's last appearance on Late Night. Richter leaned forward in his seat abruptly, as if to defend himself. "It's not my last time on the show," he protested. "I'll be back every now and then, to push that, that outdoor grill I'll be selling."

It remains to be seen how Conan will fill the gap left by Richter. The show will have lost its best remote correspondent, and one of its essential sketch players. Consider the famous recasting of the Barbara Walters' Monica Lewinsky interview, with Richter spliced into the Lewinsky shots. (Walters: "How did it get so intense so fast?" Richter: "I just said to him, 'I'm a grown man, you're a grown man -- let's get it on!'") The sketch was funny the second you got the premise, in a way that it would never have been with Conan -- or Max Weinberg, for that matter -- playing Monica. As for Richter's role on the couch, it's hard to imagine anyone taking that place, though our own personal preference is to see Triumph the Insult Comic Dog occupy the role, or better yet, his genius master, Robert Smigel. But whoever does replace Richter will at least have the benefit of a sidekick worth emulating, for arguably the first time in talk-show history. Let's just hope that there's more in Richter's future than Cabin Boy II.