Tanjiro:

Is he the first to know? No.

For a long time, he'll dismiss his feelings as friendship, mostly because he genuinely cares for everyone. To him, his feelings aren't as different as they would be for Muichiro, for example. It takes someone else (probably Zenitsu) to point it out for him to realize.

Once he notices, everything changes for him. He now sees some of your interactions in a different light and he finds it harder to stop himself from thinking about you now. He realizes that the strange scent of longing he's been smelling from you was your romantic feelings. He'll blush easier around you and will try to find more time to spend with you.

He'd probably go to his friends for advice on what to do. Zenitsu is genuinely serious about romantic matters and is likely to give him advice based on what he knows (it's not the best, but hey, it's a start). Inosuke hardly knows a thing about dating, so he'd be clueless. Nezuko is another good candidate, as she's been able to observe how her brother's demeanor has changed around you and although she can't speak as well yet, she's still able to read and write. She knows you well and is the one to give him the best advice of all three.

Nezuko:

Is she the first to know? She's definitely one of the first.

She began to question her feelings towards you a while ago with some confusion, seeing as you're both girls (and she'd never given much thought towards a relationship with a woman before). Eventually, though, she accepted her feelings confidently; she loved you, and you accepted her without hostility.

Her brother was one of the first to notice her feelings, and even encouraged her to act on them. Tanjiro trusts you with his life, and he knows that his little sister would be happy and safe with you. It doesn't mean he doesn't feel a little bit uneasy about your relationship, what with his sister's current form, but if Nezuko is happy, he's happy. Zenitsu will be dejected about Nezuko's feelings, but in the end, he'd support you two (he's a bit of a pervert). Inosuke doesn't really get it, but he doesn't care. He'll support his 'underlings' regardless.

The one she'd probably go to would be Tanjiro. He would be the one to give the best advice, having known her for his entire life and being good friends with you. He knows what you like (having had to listen to your fawning over Nezuko's cuteness and having listened to your feelings first), so advising her on what to do to win you over is a piece of cake.

Zenitsu:

Is he the first to know? Do you even need to ask?

Zenitsu is quite the romantic, and although he tends to chase skirts sometimes, he knows love when he feels it. And when he realizes that he loves you, he's more than a little nervous to breech the topic with you.

He'd probably go to Tanjiro first to beg him for advice on what to do. He's been in love before, but he's scared that he'll push you away -- he has trust issues, with good reason. Tanjiro would assure him that he should just be himself and you'll accept him (he may be a bit skeptical at this, but he trusts his friend enough to try it).

He'd probably go to Nezuko next to apologize for being so clingy and to tell her about his feelings for you. Nezuko has known for a while now, so she supports both of you. She'll happily listen to Zenitsu gush about how amazing you are, as they've grown closer in recent times.

Inosuke:

Is he the first to know? No.

Inosuke would be the type of person to mistake his romantic inclinations as being protective over a 'subordinate' or ally. Like Tanjiro, he won't give much thought to crushing on you because it hasn't crossed his mind. It'll take someone else pointing it out to him for it to hit him. Usually it's Zenitsu or Tanjiro suddenly asking him if he has a crush on you.

When he does realize it, it changes a few things. He'll grow more protective of you in the presence of others he deems threatening, not hesitating to provoke certain people to fight. It doesn't matter if they're a man or a woman -- if he thinks they might steal you away, he's announcing a pig assault. It's simultaneously a little sweet and pretty annoying, but at least he's not being a total tsundere about it (cough cough, Sanemi). Rather, he'd be vocal -- "She's mine, back off!" would be his go-to phrase for any uppity demons looking to make a meal of you.

He would probably go to the others in the squad for help if they approached him about it first. Inosuke would probably keep branding it as a protectiveness over an ally unless someone figuratively bonked him over the head (or literally, in Tanjiro's case) with the knowledge of "Hey idiot, you have a crush on her." Still, the others support it. Tanjiro is just relieved that Inosuke doesn't perceive Nezuko as a threat to his crush.

Giyuu:

Is he the first to know? Probably not.

Giyuu has the unhealthy habit of pushing his feelings away so that he doesn't have to get closer to people and see them get hurt. That being said, when he first realizes (possibly with the help of another pillar talking to him about it), he may push you away for a while to collect his thoughts and form a plan of action.

Of course, the first and foremost thing would be to speak to someone about it. He may seek the advice of Urokodaki, mainly because the man's known him for years and he might have some wisdom to shed on something like this. He may also speak to your parents about this. He's perfectly aware that he may not be the best man for you, but he does want to try his best to make you happy. At the very least, your parents know he can keep you safe.

Don't expect him to make a move right away. Giyuu needs time to come to terms with his feelings and accept the fact that he's crushing on you. He doesn't want you to get hurt because of him, and the gut reaction to push you away means he'll be a bit dodgy for a while. But once he accepts the fact and decides to give it a try, he'll come back a little softer towards you.

Kyojuro:

Is he the first to know? Pretty much.

He starts to see the differences between caring for you as a friend and caring for you as a potential partner right away. The switch may be a little confusing, but he's all for letting you know that he likes you -- given you feel the same. He may be more watchful for a while to make certain, but once he knows, he's ecstatic. He's a jovial guy, and he can be passionate when he wants to be. He'll make sure you know the dynamic has shifted.

If he ever needed to go to others for advice on romantic gestures and such, he may try Shinobu and Mitsuri, seeing as, one, Mitsuri is a Breath of Love user and quite the romantic, and two, they're both women. Mitsuri would be  e c s t a t i c  that Kyojuro's crushing on you and would give him all the advice he wanted. Shinobu, while more reserved about it, would be very happy for you both, seeing as you two had become friends during your training.

Muichiro:

Is he the first to know? Maybe not.

He's certainly not the last, but he's not the first -- it's likely that someone else noticed way before he did and decided to keep quiet until he figured things out. Once he does realize, though, it changes the way he sees you and interacts with you. Muichiro is a baby teddy bear to those he's close to, and you'll see that sweet side of him come out full-force when he realizes that he loves you.

If he had to pick, he'd probably go to Mitsuri or Tanjiro for advice -- Mitsuri because, love pillar, and Tanjiro because he's the closest thing he has to a friend so far. Mitsuri would be overjoyed that Muichiro found someone he's interested in and would gush forth with all sorts of advice for a teenage paramour. Tanjiro would be happy for him and would encourage him to act on his feelings (having smelled a difference in your emotions long before). Tanjiro is a friend to everyone, so it's inevitable that either or both of you would've come to him for help.

Shinobu:

Is she the first to know? Most likely.

While she's mastered hiding her deeper emotions behind a facade of smiles and honeyed words, Shinobu isn't a fool when it comes to her love for her friends and family. When she suddenly realizes that she may like you more than a friend (and even more than a best friend), she needs to take time to think on it. It's been a long time since she's felt a deep, deep bond with someone alive, and she intents to treat that with the utmost seriousness. It's likely that she'd seek you out once she's satisfied with her feelings, telling you outright with that same placid smile on her face. Some of that is her previous brash personality shining through, although she would be nervous about you rejecting her bold move.

If she had to go to someone for advice, she may get it out of Mitsuri. The love pillar is a romantic person, and as such would be the perfect girl for romantic advice. She trusts Mitsuri with this much at least, and the girl is always happy to help with prospective couples. (In very rare instances, Tengen may be able to help too. Slayers and kunoichi aren't quite so different in many ways, and the man has three wives who can spill more grown-up knowledge if one so desires.)

Gyomei:

Is he the first to know? Probably not.

It took a while for him to trust you, although your bright personality and unwavering kindness certainly sped up the process. For him to go from caring about you as a friend and ally to caring about you as a prospective romantic partner, it'll take a while. Chances are, others will get keyed into his changes of behavior around you quicker than he will himself.

It might be your parents tentatively approaching him about it during one of his stays. They already knew about your feelings towards the gentle giant and would want to hear his perspective. From then, he would come to realize just how far your relationship had come, and would confirm that yes, he has developed a crush on you. He would be uncertain of how to proceed -- there was quite the difference between you two, and not just in height. But with your parents' encouragement, plus his own inner soul-searching, he would come to a decision to try.

Sanemi:

Is he the first to know? Well, the short answer is a resounding No.

Trust me, it'll take a while for him to be able to separate his protective feelings towards you from his protective feelings towards his peers. To him, family is family -- he doesn't have time to play nice and settle down with a family yet. If one asked him, he'd likely shove it aside and move on. He doesn't want to have unnecessary attachments, because they'll only get hurt in the end... or so he thinks.

It'll take a truth bomb for him to finally realize that what he's feeling for you goes deeper than once thought. Perhaps someone pointed out all of the nuances of your behavior around each other. Maybe someone sparred the knowledge out of him. Either way, Sanemi is suddenly struck with the realization; he has a crush on you, and he's wholly unprepared for it.

Expect him to avoid and/or outright antagonize you to get you to go away for a while. He's likely scared of these feelings, because above all else, he doesn't want you to suffer because of him. He doesn't want to drive you away because of the way he expresses himself, and most likely he's frustrated that he can't tell you in any better way. He wants to keep you at a distance, much like Genya, but he recognizes that that's impossible in the end. While he hates seeing you hurt and upset, treating you like his little brother wouldn't work out because you're just too damn stubborn.

When he finally comes to terms with things and is ready to break it to you, expect him to be awkward in the beginning. He's still learning and coming to terms with his feelings for you and is trying to break the habit of being cold and dismissive. He really does care for you, but his years of shoving people away has made it difficult for him to keep anyone close for long, even if he wants to.

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A/N: Y'all, I'm so bi for Uzui and his wives. A beautiful man with beautiful partners? Yes please.