Though her father, Carlton Duquesne, ordered his hired bodyguards to protect her at all times, Kathy did not have a good relationship with him, for she blamed him for the death of her mother, who was killed in the crossfire of an attempt on Carlton's life.
Kathy's mother was a gifted artist, a skill her daughter inherited. Kathy also competed in many sporting competitions such as tennis matches, marathons, etc. However, Kathy was more interested in art and took this skill, enrolling in art classes, where she met Sonia Alcana in one of them. Along with Alcana and Dr. Roxanne Ballantine, Kathy devised a scheme to enact revenge on her father, Rupert Thorne, and the Penguin, the latter two receiving grudges from Alcana and Dr. Ballantine. Each of the three women brought different assets to the plan: Kathy financed the plan, Roxy provided technical knowledge, and Sonia helped to train the women in hand-to-hand combat. The three would take turns becoming Batwoman, taking down Thorne, Penguin, and Duquesne's weapon smuggling racket one step at a time. Kathy also kept up the pretense of a girl with too much time and money on her life to conceal her secret.
Batwoman had made her first appearance when she hijacked a truck which was supposedly transporting cars, when actually it was transporting the first shipment of weapons that Penguin and Thorne were selling to Kaznia. She was able to stop them, but her ruthless ways made her an enemy of Batman. A day later, Kathy had just returned from shopping with her father's bodyguards and had returned home to hear her father trying to figure out who had revealed their operations with transporting the weapons. Carlton angrily asked Kathy where she was, Kathy however reassured her that she brought his bodyguards with her as they were carrying her newly bought items. Once they left, Kathy informed her father of her day and tried to ask for his own day in return. However, he avoided her question and told her to tell him whenever she would be going out again. Kathy understood and asked who was her father just speaking to, however he only said that it was nothing for her to worry about. Getting no further answers, Kathy left.
That night, Kathy as Batwoman, had sneaked into Penguin's factory where she set a bomb to go off in the underground secret base where the weapons were being created. However, she had been spotted and was hiding away from Penguin's goons until they had surrounded her. She was captured until Batman and Robin had arrived and helped fight them off. During the chaos, she alerted Batman to the bomb she had planted, but at that moment it went off causing the entire factory to be destroyed. She along with Batman, Robin, and Penguin's goons escaped safely. She then met up with Batman and Robin where she revealed to them about the factory's true agenda, while also revealing that she had merely begun.
She started to leave, and Batman stopped her to ask for her true identity and motive for this event, but she only told him to figure it out himself and escaped. Batman followed Kathy back to her home and immediately suspected her of being Batwoman. The next morning, Kathy went shopping with her father's bodyguards protecting her. Unaware to Kathy, Bruce Wayne and Alfred Pennyworth had followed her, trying to prove that she is in fact Batwoman. However, finding no evidence, Bruce followed her into a shopping mall, hoping to better prove his theory. Kathy was trying on clothes and when her bodyguards weren't looking, she met Bruce and got him to help her escape from them.
Kathy and Bruce got to her car and escaped with Alfred following behind them, though he soon lost track of them once Kathy realized that he was following them. Later, Kathy took Bruce to her favourite spot, where she and her mother use to come out to paint. Kathy and Bruce got to know each other better with Kathy even revealing to him her mother's tragic death and that she wished that she came from a respectable family. She almost fell off the cliff, but was saved by Bruce. Saddened, Kathy had a moment of grief with Bruce comforting her and at that moment she and Bruce developed feelings for each other. However, the moment was interrupted when her bodyguards arrived. Kathy gave Bruce the keys to her car, gave him a quick kiss goodbye and left.
Later, Kathy overheard her father upset from the news that Penguin and Rupert Throne had decided to replace him as muscle for their operation. Wanting to get away from it all, Kathy snuck away from home and met up with Bruce again at Rocky Ballantine's apartment, where she had invited Bruce on a date and got to "meet" Rocky. Later that night, Kathy and Bruce went to the Iceberg Lounge, where many men there flirted with Kathy. Bruce discovered that Kathy knew the Penguin personally. Once he left, Bruce confronted Kathy over the game she had been playing, pointing out that it seems that she wants to get her father arrested. Upset with Bruce, she left for the ladies' room.
Soon the Iceberg Lounge was attacked when Batwoman arrived seeking to discover who Penguin had replaced Carlton with. Bruce then suited up and helped Batwoman. She soon escaped and right after that, Batman noticed Kathy in the crowd and was shocked to discover he was wrong about her being Batwoman. After the incident, Bruce escorted Kathy home. Kathy then began to apologize and admitted to Bruce that he was right about her. However, Bruce instead admitted that he was wrong and apologized, while also admitting that he had a wonderful time with her. Bruce and Kathy were about to share their first kiss when they were interrupted by one of the bodyguards who alerted Kathy that her father wanted to see her. Kathy said goodbye to Bruce, leaving him with a smile on his face.
Later at their secret hideout, Kathy met up with Rocky and Sonia, where both girls expressed to Sonia their worries over Batman exposing the truth of their operation. However, Sonia reassured the girls and informed them that Penguin and Thorne's smuggling operation was going down that very night. Kathy was Batwoman for the mission, and Sonia and Rocky informed her of their plan: she was to place a charge aboard their luxury cruise liner, taking the weapons and, more importantly, everyone on board with them. Kathy was ready to go, until she spotted the newspaper that had a photograph of Bruce. Kathy had a slight moment of doubt but shook it off and left for the mission.
Kathy as Batwoman used her Bat-glider and was able to sneak onto the ship and was about to set the bomb, until she was caught by Bane who had easily overpowered her and rendered her unconscious. Later, Bane informed Penguin, Thorne and Carlton of Batwoman's capture and called them to witness her unmasking. Once they had arrived, Bane revealed her true identity as Kathy, much to their shock. Penguin and Thorne then started to turn on Carlton believing he must have told her of their plans. Bane also further explains her use of the bomb.
Carlton then confronted Kathy over how she could do this to him. Kathy started to explain her motives, revealing it was easy for the man he was and also revealed of how she blamed him for her mother's death, how he kept her a prisoner of their own home and even revealed how he spread misery everywhere he went or touched. Furious, Carlton was about to attack Kathy, but stopped himself and pointed out that none of this made sense. Penguin agreed with him and mentioned how Kathy was at his club the night that Batwoman attacked, realizing she wasn't alone on this scheme and threatened her to reveal her partners with poison gas. Carlton stopped him, but was suddenly attacked by Bane and Penguin to demand that Kathy reveal her partners. Otherwise, her father's life would be lost.
However, before Kathy could explain, Batman appeared and rescued Kathy from their clutches, while at the same time, she stole the bomb. Bruce and Kathy arrived at the engine room where she had set the bomb to charge and told Batman that one way or another, this had to end. Kathy and Batman were about to escape until they found her father in chains, hanging over a swimming pool with Bane holding him at the top. While Batman dealt with Bane, Kathy dived into the pool to rescue her father, but during the chaos, the bomb went off, separating Kathy from both Batman and her father.
During the explosion, Kathy had fallen unconscious but she woke to find herself hanging onto dear life with her cape caught near a broken pole, that was starting to break apart. Carlton found Kathy, and he threw a life preserver down to Kathy and began to pull her to safety. Kathy and her father soon found a life boat, and they both escaped from the now burnt-to-a-crisp cruise liner. While escaping in the life boat, Kathy and her father witnessed Sonia, Rocky, and Batman escaping from the nearly sunken cruise ship.
Sometime later, Kathy and Rocky met up, where Kathy revealed that she and her father are now on better terms as they now talk to each other and even revealed how he plans to testify against Penguin and Thorne. Kathy asked Rocky how Bruce was doing as she hasn't heard from him since the newspapers revealed the true story behind the mystery of Batwoman. Kathy then left as Kevin and Rocky happily reunited. She spotted Bruce waiting for her, to return her car keys from earlier. Kathy was surprised to see him again and asked what he was doing here. Bruce revealed that while things were peaceful, he admitted that it made him realize how much truly missed spending time with Kathy. She was touched, but informed Bruce that she has now changed and plans to live a life full of respect, but with an explosion now and then as she and Bruce drove off into Gotham.
Kathy's name sounds similar to the original Batwoman's name, Kathy Kane, as it's pronounced "Du-kane". According to storywriter Alan Burnett, this is due to the fact that DC Comics requested they not use "Kathy Kane", as the film's Batwoman has a slight villainous side. Burnett admitted he liked the idea, as it would help keep people from guessing the solution of the mystery by adding a red herring clue.