Here's what I think a week with Finana would be like:
Finana would only target you after you've been through something that has you down; losing a loved one, breaking up with your SO, maybe even just getting rejected or having a bad day. If she was being especially pushy I could see her going for guys that aren't even single, as long as she believed she was truly a better fit for them or they were in a bad relationship. As long as it's something she views as a challenge, she'd be into going after you.
This would arguably be the only day that could be considered good for your mental health. She would simply be trying to get to know you and your troubles. She'd let you vent to her as much as you wish and would relate to you however she could. This whole day would be spent just making you comfortable around her and trusting of her. If at the end of the day you're thinking of Finana, then she's done her job well.
This would be a continuation of the previous day at first, but she would turn up the teasing/flirting. She'd dress a bit slutty even though she acts as if you're just a friend. At the same time, she'd still question you about your troubles, even if you might not be wanting to discuss them anymore. Her end goal is to get you riled up and unable to think straight. She'll try to spend the night at your place; if you accept, then she'll find a way to push you over the edge, and you'll end up spending that whole night using her as a sexual outlet for your frustrations.
Having successfully slept with you the previous night, and knowing you'll be tired from staying up later than you might be used to, she'll spend this day just hanging out with you as your live-in fucktoy. She'll still act like you're just a friend; she'll want to play games, talk about anime, etc. But she'll also act super casual when it's clear you're thinking of fucking her and she'll let you do so without a second of hesitation on her part. She's letting you continue to use her as an outlet however you want, but at no point will she hint at any romantic feelings. No "I love you"s, just sex.
At this point it's going to be incredibly hard to not catch feelings of some sort; you're getting along, she's incredibly cute, she seemingly shares all your kinks, and is willing to do whatever it takes to make you happy. You think you're falling in love with her, but really you're just becoming addicted to having an unrealistic relationship where a girl is willing to provide you with extremely high amounts of sex & companionship. In the heat of the moment, during more passionate sex, you'll finally tell her that you love her and she'll reciprocate without thinking much of how heavy those words really are.
This is about the time when things start going downhill, but because of her methods, neither of you will ever really realize how unhealthy the relationship is. You'll become more and more obsessed with her, while Finana will, at least subconsciously, already be thinking of her next challenge to conquer. She may have said she loved you the day before, but she won't act much differently towards you. You can still use her however you want though, so you won't notice that she may head home an hour early or that she didn't talk to you as much throughout the day.
She's already pulling away from you, but you're too blinded by your lustful obsession to notice. You spend the whole day thinking about her. All you want is to hear her say "I love you" again. In a reversal of Day 3, you will be the one spending your whole day trying to please her. You'll try to impress her by dressing up as nicely as possible, you'll pay for an expensive meal, you'll ask her about herself the whole day without even mentioning your own thoughts on things, and when you both fuck you'll be entirely focused on pleasing her as best you can.
At this point, you've been successfully converted into a fully dedicated Ryuguard that Finana can call upon whenever she wants. You can't see anything that she does as doing anything wrong. You think you're in love and that you have a chance with her, but the reality is that you just want this one week to last forever, no matter how unsustainable it is. If you're lucky, you'll be a favorite servant of hers that she calls upon for kinky sex on the regular, if you're unlucky then you'll just be a distant orbiter who defends her blindly from others.
Finana's version of fixing others isn't to make you a better person for yourself; it's to make you a better person for HERself. She likes tackling challenges, she has too high standards for a realistic romantic partner, and she's too warped by media to realize when she's being manipulative. She can't help being a siren; it's all she knows how to be and it's too much fun for her to give up. Be wary of her, and try not to fall for her siren song, no matter how tempting it is.