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). Kei lost his feminine parts (entire female reproductive system), therefore believing he lost his entire female identity with it, and begins 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 entire female reproductive system, believing that he was no longer a woman, and blocks out the former feelings he had for Black Jack when he was a woman. However, it's shown years after the surgery that they remain good friends.
He is a lean man with short black hair styled in a rather wispy fashion, wearing trim glasses with brown eyes. He wears a suit with a blue button collared shirt underneath and tied with a yellow tie.
- Neither Kei nor his previous female indentity, Megumi, do not appear 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 of Black Jack 21 as the patient Black Jack treats (though it doesn't explain their history together but does hint at it) by showing Black Jack insisting to operate alone on her, which angers the head doctors at the hospital, who responded by revoking Black Jack's medical license.