Found the following article online, use it to answer the question: Who is keeping Kimberly locked up?  The film opens with Stanley on his boat in the middle of Summit Lake. After speaking with his agent, Regan over the phone, Stanley accesses a hidden room in his house revealing a young woman who is chained to the floor by her ankle.  He greets the woman and calls her Kimberly as he places a plate of food in front of her.  When he leaves the room, Kimberly walks over to the mirror and begins to cry, unaware that Stanley is studying her on the other side. Afterwards, Stanley brings her a fresh change of clothes and instructs her to dress, before gagging her with an orange and binding her hands behind her back.  He forcibly escorts her onto his boat and drives out to the middle of Summit Lake, where he ties a cement block around her ankles.  Curious about her predicament, he asks Kimberly how she feels, but the terrified woman does not respond. Stanley then pushes her into the lake, but promptly pulls her back up by the hair to study her expression before letting her sink to the bottom.   Later, Stanley returns home to resume working on his script using the details he had gleaned from his latest victim.  The next morning, he dives to the bottom of Summit Lake to tend his "garden", which consists of several women kidnapped and drowned in a similar fashion.  Seeking yet another victim, Stanley discovers Mallory, a young woman working at a movie theater.  Intrigued by her self-confessed fear of water, Stanley follows Mallory after she leaves work and swerves in front of her, slamming the brakes on his van and forcing their vehicles to collide.
The answer to this question is:
Stanley