Me? A Villain? I'm shocked.

Is MARRIAGETOXIN a BL?

From a marketing standpoint, MARRIAGETOXIN has never been and will never be labeled as a BL. It is a battle shounen through and through. However, just because it's not a BL does not mean it's not queer.

And usually when people ask this question what they're really trying to ask is, "Will Gero and Kinosaki get together?" I can't predict the future, and the manga is still ongoing. That being said, at this point in the manga, it is HEAVILY implied that Gero and Kinosaki are in love with each other.

2. Is Kinosaki trans?

I'm not going to answer this as a yes/no question, but give some clarity about what they have/haven't said.

For starters, Kinosaki fits cleanly into the social identity of an otokonoko (オトコの娘, literally "male daughter"), which is sometimes translated as femboy. That's not a horrible translation, but those two words are not 1:1. "Otokonoko" is an identity that excludes neither binary cis men NOR binary trans women. Saying something like "they can't be trans, they're otokonoko" is not useful because those ideas are NOT mutually exclusive. Kinosaki says in the first episode/first chapter that they are a guy (男 otoko). They could be referring to their gender identity, their assigned gender at birth, or both. I want to emphasize that it is not unusual for even binary trans women in Japanese to use phrases identifying themselves as "men," "men who want to become women," or "men with the heart/mind of a woman," so I don't think it's fair to say that that wholly negates a transfem reading of Kinosaki.

Kinosaki presents as female and is regularly assumed to be a (presumably cis) woman and shows no discomfort towards that. We don't see Kinosaki presenting as male much, but they haven't said they have any discomfort towards that either.

Kinosaki's personal pronoun is watashi, which is female-leaning in casual contexts. Kinosaki's first name, Mei, is also very feminine, and might therefore be a chosen name or pseudonym (it's also written in katakana, which may indicate a pseudonym in other media, but many names are written in katakana in MARRIAGETOXIN so that's not particularly notable). Kinosaki's way of speaking is a bit feminine-leaning but mostly gender neutral.

Kinosaki says they enjoy being attractive to "both sides" in Japanese.

So....basically, it's not too hard to substantiate any cis man, trans woman, x-gender, or unlabelled reading of Kinosaki with canon points. Kinosaki is also an unreliable narrator with some secrets, so keep that in mind.

3. I've heard that Kinosaki was revealed to be "a girl all along," is that true?

This is a common statement on Twitter. If you're asking about Kinosaki being a trans woman, see above. If you're asking about Kinosaki being a a cis woman, anyone who says that Kinosaki is a cis woman in the manga is lying out of their ass. Some people believe that Kinosaki will be revealed to be a cis woman, or that there are "hints" that Kinosaki is a cis woman. Again, I can't predict the future, but "hints" is a massive reach.

Most of the "argument" comes down to the fact that Kinosaki's chest has been obscured very occasionally:

There is a spoilery, non-gender-related reason for this. Some people also point out that, even after that reason is revealed, their nipples are still obscured and like...listen, it's a Shonen Jump title, and seeing a very feminine person with exposed nipples can lead to controversy. Regardless, it's clear that Kinosaki's chest is flat: