Lauren is respected in her profession, she's shown to be very successful and a total turnaround from Duckman's path. When we first see Lauren, she is entertaining a crowd of people with a few anecdotes, and that's when Duckman makes a big interruption.
She nominated Duckman for an award that he was to believe as a "Lifetime Achievement Award", and Lauren asked if the two could put their school rivalry to rest.
But Lauren's vindictive side comes to light during the big ceremony, when Duckman takes his stand on the pedestal.
To everyone's mockery of him, Duckman rereads the text on the trophy, only to find it actually spells "Lifetime Underachievement Award". The whole thing turned out to be a monumental practical joke at Duckman's expense, with Lauren getting a good laugh in.
The next day, Duckman tries to redeem his abilities by acing a contest for the grand prize of $10,000 and a trophy inscribed "Best Dick '97". He winds up getting Cornfed to complete the test for him while he distracted Lauren with his Groucho Marx bit, yet still Lauren knew the answers were Cornfed's doing.
Lauren's cruelty is shown further when she kidnaps Cornfed, and uses that to force Duckman to show his actual skills as a detective. When he obviously screws it up, he falls into a bitter depression at a local bar.
Lauren happened to be at the same bar, equally downhearted. Lauren reveals to Duckman how she kidnapped Cornfed simply to humiliate him, as she dreamt of the day when she'd make him look like more of a fool, and how happy that'd make her. But, it didn't.
All she felt when she saw Duckman at his lowest, all she felt was sympathy, not satisfaction. Lauren realized that she felt some sort of affection for Duckman, and this had made her sick with herself for feeling something for a guy like him. That's why it looked like she hated him, and this sparked their initial rivalry. She was sublimating her feelings for Duckman into hate.
Her decades-old crush was given closure when the two decide to embark on a night out, where they could spend the night at one of the most glorious cities on Earth and where Duckman can use some stolen credit cards.