Kei Kisaragi is Black Jack's old friend. He used to be Megumi, Black Jack's former love interest who underwent surgery to cure herself of her ovarian cancer. Black Jack performed the surgery but he lost his feminine parts, therefore losing his female identity, and began to live his life as a man.
He appears to be a kind person who respects Black Jack as a friend but wishes to forget about his past as a woman, having chosen to become a ship's doctor because it was a man's job.
He was Megumi, and Black Jack's girlfriend before he had surgery due to having ovarian cancer which forced Black Jack to remove his feminine parts, making him a man and destroying his female feelings, including those he held for Black Jack when he was a woman, however it's shown years after the surgery that they remained good friends.
He is a lean man with short black hair wearing trim glasses with brown eyes. He wears a suit with a blue button collared shirt underneath and tied with a yellow tie.
- Neither Megumi nor her male persona Kei make an appearance in the anime (possibly due to controversy over the presentation of his surgery as presented in the manga) but he does appear in the anime in the second opening. He also appears in the first episode o Black Jack 21 as the patient Black Jack treats (though it doesn't explain their history together but does hint at it) and how his treating him led to angering the doctor in charge of the operation who responded by revoking Black Jack's license.