Lisa Sheridan

"You need help."

- Derek Charles to Lisa Sheridan. "I know. Help me, Derek."

- Lisa Sheridan who is constantly obsessed wirh Derek Charles.

Lisa Sheridan is the main antagonist of the 2009 thriller Obsessed.

She is portrayed by Ali Larter.

Biography
Derek Charles is the Executive Vice President of Gage Bendix, a finance company. Derek and his wife, Sharon have an infant son, Kyle (Nathan and Nicolas Myers). While at work, Derek briefly flirts with a new temp named Lisa Sheridan, who later attempt to seduce him throughout the film. Derek repeatedly rejects her, but Lisa continues to advance on him, and attempts to have sex with him at the no-spouse Christmas party and flashes him in his car. Derek intends to report Lisa to his firm's human resource management, but learns that she has quit her job. Derek and his workmates visit a resort for a conference, where he spots Lisa. He confronts her, who spikes  his drink. Incapacitated, Derek is somewhat helpless when Lisa follows him into his hotel room and kisses him. He confronts Lisa again the following day, and hours later discovers her lying in his bed after attempting suicide through drug overdose. After repeated attempts to reach Derek on his phone, Sharon finds Derek at the hospital, and suspects that he and Lisa had an affair, as Lisa claims. Detective Monica Reese questions Derek and becomes skeptical of Lisa's claims, and informs Derek of her belief in him. Sharon kicks Derek out of their house, and Derek moves into a separate apartment.

While Derek and Sharon are dining out, Lisa breaks in their house and tricks the babysitter Samantha into letting her in under the pretense of being one of Sharon's friends, and flees with Kyle. When Derek and Sharon return home after dinner, they discover that Kyle has been abducted. Derek goes to his car with the intent to pursue Lisa, only to find Kyle sitting safely in the back seat. Derek and Sharon immediately take Kyle to the hospital for a check-up. When Derek and Sharon return home after checking-up Kyle, they find Lisa has trashed their bedroom and removed Sharon's face from their family portrait. Sharon leaves a threatening voice message on Lisa's phone, and she and Derek set up a home alarm system. Lisa learns that Derek and Sharon will be away from town, with Sharon leaving that afternoon and Derek the next day. While Sharon is on her way to pick up Kyle, she realizes that she forgot to set the alarm system and returns home. Meanwhile, Lisa breaks into Derek and Sharon's house again and decorates the master bed with rose petals. While setting the alarm, Sharon hears Lisa in the bedroom. Sharon tells Lisa that she is calling the police, but Lisa proves to be far stronger and more dangerous than she anticipated and easily tackles her to the floor and Sharon and Lisa engage in a fistfight. Sharon is aggressive and uses her size advantage while Lisa targets Sharons throat. Lisa gains the upper hand by elbowing Sharon in the face and attempting to force her over the stair banister while choking her with both hands. In desperation, Sharon lunges forward and Sharon and Lisa engage in a struggle where Lisa, refusing to let go of her throat manages to overpower and wrestle Sharon down the stairs, temporarily knocking her out. Derek calls his house and Lisa answers and later calls Detective Reese and immediately leaves his office.

Lisa runs to the attic, and Sharon pursues her. Sharon leads Lisa to a weak spot in the attic floor, where Lisa falls through. Sharon reaches out and attempt to take Lisa's hand to lift her up, but Lisa pulls Sharon down with her instead of accepting her help. Seeing that the floor is starting to buckle, Sharon pries Lisa off of her arm. Lisa falls onto a chandelier, breaking her fall, but lets go and falls onto the glass table below. Lisa opens her eyes, only to have the chandelier fall, which finally kills Lisa. Derek and Detective Reese arrives as Sharon comes out of the front door of the house. The film ends when Derek and Sharon embrace each other as Detective Reese enters their house to investigate Lisa's actions.