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.