Like a CATWOMAN on a hot tin roff, Michelle Pfeiffer is sizzling this summer. As the slinky, kinky, whip-snapping, wisecracking antiheroine of Batman Returns, she has helped transform the year's most eagerly anticipated movie into one of the biggest blockbusters of all time (more than $100 million and purring) and guaranteed herself superstar status in the bargain. Yet despite a glamorous image that keeps her in the public eye (she's been one of PEOPLE's 50 Most Beautiful People in the World for three years running), Pfeiffer, 34, remains a mystery woman offscreen. Playing Catwoman, she has said, "I have to be frivolous and off-the-cuff and over-the-top, and it isn't my nature to be any of those things."

Indeed the unmasked Michelle appears to be a lot closer in personality to Catwoman's mousy, inhibited alter ego, Selina Kyle. "The way my mouth curls up and my nose tilts," Pfeiffer once joked, self-deprecatingly, "I should be cast as Howard the Duck." This other Michelle downplays her beauty by routinely pulling her hair back into a ponytail and wearing no makeup. Often hidden, in the interest of privacy, under a floppy hat, behind sunglasses and in T-shirt and jeans, she certainly makes no effort to look like a movie goddess.

Nor to seek happiness with another matinee idol. She already tried that in a nine-year marriage to thirtysomething's hunky Peter Horton (they separated in 1988 and divorced two years later) and in two brief flings in 1988 with Batman's Michael Keaton and her Dangerous Liaisons costar John Malkovich. But for the past three years, Pfeiffer has kept company with Fisher Stevens, a 5 ft.8 in. character actor (Short Circuit 2, My Science Project) who specializes in playing nerds, wears $40 thrift-shop suits and, at 28, is six years her junior. Considering some of the fabulous men she has starred opposite (Sean Connery, Mel Gibson and Jack Nicholson, among them), Pfeiffer knows that, at first blush, her choice of steady may seem unusual. "Fisher is not your standard leading-man type," she has admitted, "but I adore him."

By all accounts there is much to admire in Stevens. "Fisher is terribly creative, dedicated, honest and kind," says Kenneth Johnson, who directed him in Short Circuit 2. "He's the kind of guy you can't not like. I wasn't surprised when Michelle was drawn to him. I wouldn't be surprised if Elizabeth Taylor was."

Pfeiffer and Stevens met three years ago, when both were appearing in a New York Shakespeare Festival production of Twelfth Night. She was making her New York stage debut as the desirable countess Olivia; he was her foolish suitor, Sir Andrew Aguecheek. Their real-life courtship had a shaky first act. Stevens, who at 18 had changed his name from Steven Fisher (already used by another actor) and had previously been linked with Helen (Supergirl) Slater, whisked Pfeiffer to Paris for what was supposed to be a romantic interlude. Just as they arrived, however, Stevens fell "violently ill" with the flu, recounts Johnson. "How would you feel if you were with a beautiful, talented person in Paris and you were sick? I remember him saying that she was terrific about the illness." Their romance survived that crisis, and they have been loyally winging ever since between his co-op in Manhattan, her hacienda-style home in Santa Monica and their far-flung locations.

Pfeiffer seems determined to make this relationship work. "When she sets her goals in a direction, look out," says her father, Dick, 59, who now regrets his skepticism back when she was a struggling actress in her 20s. "Aah, Michelle," he told her then, "you'll end up a broken-down housewife with a kid on each hip."

Pfeiffer's dad thinks his prediction made her all the more determined to be a star. The second oldest of four children raised by Dick, a heating contractor, and his wife, Donna, 60, a homemaker, in Midway City, Calif., about eight miles from Disneyland, Michelle herself recalls being "completely out of control" while growing up. She totaled her first car (a red '65 Mustang) at 16 and skipped classes at Fountain Valley High School (where she maintained a solid B average) to hang out at nearby Huntington Beach with the surfers and lifeguards. After toiling as a supermarket clerk and studying to be a courtroom stenographer, Michelle entered her first and only beauty pageant and became Miss Orange County 1978. That landed her an agent and a bit part on Fantasy Island. A year later, at 20, she won her first series role, a jiggly undergrad named the Bombshell on Delta House, ABC's short-lived rip-off of Animal House.

Costar Bruce McGill remembers Pfeiffer as "young, very green, but very willing." He also took a paternalistic interest in her. "I never used to think she ate enough," he says. "She ate asparagus for five days and nothing else."

While an acting student in L.A., Pfeiffer joined a vegetarian cult that she later claimed "brainwashed" her. She was rescued, she has said, by Horton, a classmate, whom she married in 1981. Dick Pfeiffer, though still friendly with his former son-in-law, remembers him as "a very domineering person. One night we were all in a limo on our way to The Tonight Show. Michelle was a guest, and during the ride Peter was instructing her on what to say. She was very obedient to him at the time. After she left him," says her dad, "her career took off" -- with acclaimed performances in Married to the Mob, The Witches of Eastwick and The Fabulous Baker Boys.

She also came into her own personally. "After Peter Horton, Fisher seemed like a strange turnaround for Michelle," Dick Pfeiffer concedes. "But you know what she told me? 'Dad, Fisher makes me laugh. The others made me cry.' " Fisher, adds Dick, is "real good to Michelle and fun to be around."

"Fisher has a great sense of humor," confirms his mother, Sally Fisher, 49, a Los Angeles painter and AIDS activist who raised her son alone in Manhattan after being divorced from his father, Norman, a Chicago furniture executive. And she thinks the apparent differences between Fisher and Michelle are merely skin-deep. "Fisher would not be with someone just because they're beautiful," she says. "He's with Michelle because she's wonderful inside."

Looks aren't an issue, their contemporaries concur. Nor, they say, is the couple's six-year age gap a barrier. Fisher, who as a teen successfully battled Hodgkin's disease (a lymphatic cancer), is "very mature for his age. He's 28 going on 70," says David Beaird, Stevens' executive producer on his upcoming Fox series, Key West, a sort of Southern Exposure in which he plays a lottery-winning writer from New Jersey. "He comes across as sensitive and smart," continues Beaird. "And he's fascinated by people. He's interested in the busboy and the hot dog vendor. He could go to a KKK rally and walk up to anybody and find out why they got demented."

Stevens' friends, who address him as Fish (Michelle prefers Fisher), add that he's a good dancer and a competitive amateur athlete who excels at squash, basketball and touch football. "He's also a good poker player," says Dick Pfeiffer. "We play when he visits."

Michelle, in turn, goes shopping with Fisher's mother in L.A. and once skied with both of them in Taos, N.Mex., where Pfeiffer's fans inevitably discovered them dining at a lodge. Sally Fisher describes the fans' behavior as "appalling. People would come right up to the table and start arguing out loud whether that was Michelle Pfeiffer sitting in front of them," she says, ruefully. "Don't these people have a life?"

Under the circumstances, Michelle and Fisher are trying to keep their life together as private as possible. Whenever she jets down to the Key West set, says Beaird, "he's very protective of her. He stays with her, keeps the press away, keeps everyone away. Maybe that's why they're still together."

But all too often they're apart. "It's difficult," Stevens says of their cross-country relationship. "But we're working it out." Last month he flew from Wilmington, N.C., where he is shooting Super Mario Bros., a big-screen version of the popular video game, to escort Pfeiffer to the Hollywood premiere of Batman Returns. As photographers and reporters mobbed her outside Mann's Chinese Theatre, Stevens stood about 10 feet away, all but ignored and looking uncomfortable. "It's completely overwhelming," he said softly. "There are just so many people."

Which is why they prefer to stay home. Last winter, while Pfeiffer was shooting Batman Returns in L.A., she and Fisher nested in her Santa Monica spread. After an exhausting day on the set, he recalls, "she'd come home with a completely different hairstyle, depending on whom she was playing, Selina or Catwoman. It was funny." She also walked around the house practicing her feline avenger's antics (the kickboxing close-ups onscreen are all hers, though a stunt double did Catwoman's backflips). And, says Stevens, "she was constantly cracking her Catwoman whip."

When, if ever, will Pfeiffer take another crack at wedlock? "I don't know if they'll marry," says Sally Fisher. "But they're very affectionate with one another, and they think about having children."

"Michelle loves kids," says Dick Pfeiffer, sounding like a hopeful grandpa. "Her friend Kate [Guinzburg, actress Rita Gam's daughter] has a baby, and Michelle just loves to hold her." Pfeiffer herself has said she'd like to have children, but "the life part isn't right yet." That was two years ago, and now, as one half of a two-career couple, she's still working nonstop. "She likes it better when she's working," observes her dad.

That could change as quickly as Selina Kyle turns into Catwoman. Not unlike the conflicted character she plays, "I kinda feel like Michelle is torn about stardom," says her old Delta House castmate McGill. "Even when we were together, there was a part of her that would have liked to be on some dusty old ranch in New Mexico." McGill, who recently ran into Pfeiffer at an L.A. eatery, adds, "She seems a little world weary. That's unfortunate. She should be enjoying it while it's happening."