Dr. Karen Jenson is a brilliant hematologist, making the night turn in hospital when a completely burnt Quinn is brought in by paramedics. While attempting to cure him, Quinn wakes up and bites both her and her colleague Curtis Webb. In that very moment, Blade arrives to finish Quinn, who manages to escape, and he's supposed to kill her too, but she reminds him of his mother, and he decides to save her. Blade brings Karen to his base, where Whistler complains about his decision.
When Karen wakes up, Whistler acknowledges her on the existence of vampires and on their mission of vampire hunters, telling her of Blade's past too. Karen decides to come back home, and Whistler gives her a spray charged with garlic and silver pouder. Blade warns her that she'll become a vampire too, and she decides to start a research on a possible cure.
Once at home, Karen is attacked by Officer Krieger, one of Deacon Frost's familiars. She's saved by Blade, and complains about being used as a bait. She decides to be at Blade's side, both for personal security and for getting through the situation, and the two follows Krieger to a Frost's club. In here, they find out the Deacon's plan regarding La Magra, the Vampire God, and confront the obese vampire Pearl. Karen, disgusted by the vampire, kills him with a UVA torch when he has told them all the informations they needed.
Blade and Karen are then attacked by Mercury, one of Frost's lovers and generals, and her minions, but they manage to escape thanks to Whistler's arrival. Once at the base, Karen, starting from her own corrupted blood, finds a cure, and injects herself with it, healing from the infection. A short time later, Deacon Frost himself attacks the base and kidnaps her, after torturing Whistler. She's used by Frost as a bait for Blade, who sees a videotape in which she asks for help.
In Frost's base, Karen mocks him about his human origins, and tells him about her discoveries on vampire transformation, which is caused by something similar to a virus. She tells him about the cure she found, too, claiming that she doesn't care if he bites her, since she can cure herself.
When Blade is captured, Karen is brought to the temple of La Magra and thrown in a pit, where she finds Curtis Webb, who has become a ghoul. She fights him and manages to escape using the human bones found at the bottom of the pit. She goes to Blade and frees him from the iron maiden in which he was held, but she's attacked by Vanessa. After Blade kills the vampire, Karen offers her blood to him, so he can regain his strength to fight Frost. Blade nearly drains Karen of her blood but he stops himself in time. During the fight, Karen is attacked by Mercury, but she manages to kill her with Whistler's spray.
After Frost's death, when it's all over, Karen offers Blade her cure, but he refuses, wanting to preserve his powers in order to keep fighting the vampires.
Karen is quite a plucky and courageous woman, ready to any kind of situations, even the weirdest. She is a brilliant scientist, gifted enough to find a cure for the ones who have been bitten by vampires in a very short time, thus surprising even Abraham Whistler.
- Eric Brooks/Blade - Ally.
- Abraham Whistler - Ally; Deceased.
- Curtis Webb - Former Boyfriend turned Enemy
- Deacon Frost - Enemy; Deceased.
- Officer Krieger - Enemy; Deceased.
- Quinn - Enemy; Deceased.
- Mercury - Enemy and Victim.
- Vanessa Brooks - Enemy; Deceased.
- Pearl - Enemy.
Behind the scenes
- to be added
- In a deleted scene, Karen sees a shadowy figure at a distance, dressed in rags, on a rooftop. This was supposed to be an appearance of Morbius the Living Vampire, the intended villain for Blade II.
- For the Spike program, Blade: The Series, Karen Jenson was to make an appearance and later become a series regular on the show in its second season. She was planned to help Blade and Shen find Marcus and the remaining samples of the Aurora virus, while returning to settle the issues between Blade and herself. The producers of the series also provided that if the show was allowed to continue, they would have the series explain, through dialogue, her absence in the sequels to the movie franchise.