HBD....Heart Beat Disorder?
I suddenly remember a Capcom videogame character with a similar death. I think the death and reincarnation was explained in the 1st installment and the HBD was mentioned in the 3rd.
The reincarnated twin brother of Lan Hikari named Hub Hikari, if I'm right.

A 10 out of 10 on this story, Alex! ^u^

I know this is late, but that was Mamoru, Serenade's operator (it's inferred). Also, the official name has never been disclosed, but Heart Beat Disorder is the most popular theory.

Even later still, but it was both of them. Hub died as a baby from HBD and was brought back as Mega/Rockman. It was because of this that Mega reconized Mamoru's condition instantly and was able to take the actions needed to save him. I quote directly from the scene where Mamoru had an attack - "HBD is what took my life as Hub"

this was the 1st story of yours i ever read, years ago on deviantart.