From WikiGTA - The Complete Grand Theft Auto Walkthrough
Debra is the girlfriend of Floyd Hebert. Not much is known about her, but we start to learn more about her in the mission Hang Ten, when it's getting clear that Debra is cheating for awhile with a guy named Bob, which also learned her how to use a gun. In the same mission threathens the Trevor and Floyd Hebert with a gun. Floyd picks up a knife, and after that we see Trevor freaking out. Trevor walks after awhile out of her house, covered with blood. Debra and Floyd are killed by Trevor.