[Spoilers for BJ volume 1]
Well, Kei/Melgumi was one of Black Jack's many lovers over the manga, ending pretty tragically though with a sorta happy note, after her surgery they seem to not be comfortable with being a couple anymore, even if she's still a doctor oversea's. For once Black Jack is the first one to pursue, unlike the Black Queen or Pinoko, giving her an umbrella anonymously (which I thought was pretty sweet) and making sure she wasn't ever in danger. I personally really liked this story and the relationship, I was wondering what other people thought of her in contrast to the other sweethearts Black Jack gets.
Btw, was she ever in any other works by Tezuka? I don't think she was in the Star System..