Not the best reply! Close to the worst, actually. The only problem is the dynamics of the situation; forced in a way to be around each other, working together and so on.
This 50/50 stuff doesn't really work with women. And she's an ISTP, which is like... a loner in a way, someone who could do without contact with society for 6 months and it not even be a thing. A mechanic, a tomboy, a bro. So, you really could just be a bro. ISTP would be found in remote places doing difficult work, and want to do the work, not sit at a desk. Sounds like you guys work in a logging company or something. Remote and work together.
I wrote all that before your new replies. Let's see...
Probably not interested.
"If I don't like you, maybe I lose my job, so okay, thanks for letting me know, this is awkward."
Yes, if you weren't living so close, she probably wouldn't be that close a friend. She's a bro. She likes poking fun and getting her buttons pushed. She's not emotional, ISTP is one of the least emotional types. So lovey-dovey stuff is cringe for her. You're giving off sensitive vibes when she probably just wants to be manhandled.
As someone else said, assertive and asshole is what some women respond to - until they're a single parent with two children. But, in the interim, primal mating dominates the urges of high-Se users. You're likely giving her emotion, and with Fe being her inferior function, she dislikes receiving emotion. She dislikes receiving emotion - and actively avoids it. She's in her happy spot when there's no emotion being dished out. Which is why things like "I like you" don't fly. She's probably the kind of girl who would respond better to "wanna smash?"
With all this in mind, it seems like you're looking for a more emotional relationship. You don't say you're really pulled towards her, so you could just be responding to limited options and hormones from being in such close proximity. If you really cannot help dish out emotion, she's just going to keep pulling away, because inferior Fe actively moves away from emotional energy. If you cry, she's gone kind of thing.
If you wanted to be with her, you'd probably have to break down all your nice guy barriers, and do something out of your comfort zone. I could give you ideas, but they might be crude. Instead, just think of how to be laser-direct in what you want. Generally it's a pattern of behavior though, not a one-off thing. You'd have to be an assertive dick for a long period of time, one who doesn't show any emotion, to regain interest. You would need to joke around and make fun of her and play that kind of game. The friend who called her fat is higher up in her mind, because he doesn't give off emotion to her.
Which again, I don't think is your personality, and you'd really have to adapt yourself in this situation to get anywhere. You'd have to change.
The pains of too little information... people can only advise you on the information they have. So even though I thought all these signs were "Yes" in the initial post, the close proximity, position of power, and her type, all change the picture.
For your own sake, I think you again need to be laser-direct here. Otherwise it is going to eat at you. You're at the point of no return here. If you don't do anything, you've failed regardless. If you push forward through your shyness and directly state what you want (what do you want, anyway? holding hands walking down the road? what does she want? Probably mudwrestling

), at least you will know. Like the guy at the bank who just says hey I think you're hot what's your number? What does he want? Just to hook up. And she's responsive to that. If you want a friend with benefits, she's probably more responsive to that.
The only thing I can really say for certain, is that as ISTP, emotion is like an opposing magnet, and they will run from it. Everything else I'm saying is based on a world created in my mind with all these variables and attempting to see where they play out under different situations. You can't hurt her feelings, and she dislikes people who get their feelings hurt, so she's around a lot of guys, who generally have thick skin. Sensory experiences fuel her. Punch her in the arm playfully is her love language. Skydiving. Driving fast. Any sensory thing. I'm not sure your type...
Ok I did some looking at your post history, and you may have said you are ENTJ in a post. This makes sense why you guys get along well, because you dislike giving emotion actually, it makes you cringe, and she dislikes receiving emotion which makes her cringe. You guys connect really well on the idea-level - you guys both have and enjoy the same ideas and things to do and you like her ideas a lot. You both enjoy a sensory experience, experiencing the sights and sounds of the world, though she enjoys it more. I'm surprised you're in this situation actually, but it could be due to the fact that, since Fe inferior users like ISTP still need emotional energy (they just don't need a lot) and since you give off a tiny bit of emotional energy that she needs, she feels comfortable around you. She's just hanging out absorbing the little emotion she needs from you and that makes her fulfilled, where too much emotion makes her uneasy and absolute-zero emotion makes her depressed, and having to be emotional makes you uncomfortable. You probably both enjoy action/anime/scifi movies more than dramas. You guys fit well in that regard. There's no Si between you two, to fill your Se, so she's always on the look out for some sensory experience, and you are too but to a lesser degree; the thing here is, you don't provide it. You don't fill that void for her and she needs a lot of it, Se is her main way of interacting with the world. Punch her in the arm!
Your Te is going to use this information to make a winning-idea call on how to resolve this situation. Trust your Ni intuition from this point on. You really never needed help here. You just needed to be more assertive.And again, her Se is how she interacts with the world and how she feels fulfilled. Any sensory experience is good for her. Anything that makes her physically "feel", is her love language. You can do a lot with that information.
You're trying to connect to her on an emotional level with your inferior Fi, when you really need to be connecting with her on your Te/Se level. You need to ask her, "Hey, what do you think would be really fun to go do?" She's going to come up with some idea using her dominant Ti that facilitates her getting something sensory to fill her secondary Se. And you will use your Te/Ni to know what is a great place to go do that great idea she came up with. And when she sees you just want to have fun with her and don't care, all while you guys are getting this excited, sensory experience, you will grow closer. If you two have never been sky diving, that would be a first that would associate you with fun sensory experiences. But, her idea will be way better, so defer to whatever she comes up with, and apply your Te to implement that idea the best way you know how. It's what you're good at.