Calavanda

quick note first, obviously since i'm not yet caught up things are likely to change, I don't know to what degree the ideas I have conflict with canon material, like for example i've recently found out that Marco has eerily similar powers to what I've had in mind for Umi, like almost the same damn thing, and I had no idea the whole time, so yeah things are subject to change

anyway, Umi's power doesn't actually come from a devil fruit and some people believed it's the Mera Mera no Mi but that's not exactly the case, although there are some slight connections!

I'm not gonna delve too deep into Umi's past yet but to get the gist of things, I created her with Sanji and his backstory in mind, and so I created some parallels between them, by which I mean Umi had a whole lot of uncanny experiments done on her involving genetic modification/alteration, reused samples of existing devil fruits, the whole thing ya know. the abnormality and brutality of the experiments as well as another factor i'll be talking about later all caused Umi to develop an equally abnormal condition where her blood is literally boiling inside her. yes you read that right. upon exposure to air, her blood immediately lights up in flames as though you're lighting up gasoline, hence Umi is in a constant state of burning and rapid healing, no she has not felt a single moment where there was no physical pain to be felt.

see, the idea was to enhance her body to make her blood heal/regenerate almost instantaneously, and it does, but it was also a very risky and dangerous and quite absurd goal to achieve, resulting in whatever side effect we see now.

Umi is not actually a devil fruit user she's just yet another victim of human modification through scientific experiments like Sanji except unlike Sanji it was physically implemented on her years after birth. she's not a devil fruit user yet. the catch here is that, while her mother was pregnant with Umi she had eaten some blood devil fruit (? I do not have a name for ts yet Im sorry </3) her mother's dead now, meaning the devil fruit abilities can be obtained by someone else, however for years no matter who tried nobody inherited it because it was taking an extremely slow process in settling properly in Umi's body due to the modification. technically however the fact that her bloods specifically is what sets on fire is connected to the undeveloped devil fruit.