She played spunky in 1996’s ”Harriet the Spy,” irksome as Buffy the Vampire Slayer’s little sister, Dawn, and bikinilicious in last February’s big-screen romp ”Eurotrip.” Now, with a role in HBO’s ”Six Feet Under,” Michelle Trachtenberg finally gets to tap into her self-proclaimed ”inner bitch.” The 18-year-old actress plays Keith’s bodyguarding charge, a fabulously diva-esque pop tart named Celeste who frolics with bad boys like Colin Farrell but also vows to remain a virgin until marriage. Hmmm, could the show’s creator, Alan Ball, be paying homage to a certain former Mouseketeer whose name rhymes with Whitney Beers? ”Well, kind of,” says a diplomatic Trachtenberg, who debuted on June 27 and will appear in three more episodes. ”Celeste is really a combination of everyone — Britney, Christina, and Jessica Simpson.” Not to mention…herself. ”When I auditioned, I was sick; I was hardly the perfect angel,” she confesses. ”I was like, ‘I totally don’t feel well. Let’s just do this.’ Guess it worked!” Totally.