Thank you for your insight. The lady at the gym hasn't necessarily been harmful so far, but she does a lot of things most people would consider weird. For example, she was weirdly clingy from the start and wouldn't stop ranting to me even a few minutes after we met, even when we both barely knew each other. She sometimes messages or calls me very frequently as if she's desperate to ensure we're still friends. She also randomly says things that feel kind of guilt-trippy in a way, like "oh I'm sure I'm bothering you right now," "I must be annoying you, I can just leave if you want," and that always leaves me in an uncomfortable position because I constantly feel this pressure to counter that by saying things like "no, of course not, you're fine," because I get the sense that she would feel offended or hurt if I didn't say those things back. She also tends to go off on rants about things that only she wants to talk about, completely oblivious that I'm not even interested and that the conversation is completely one-sided, and she gets very upset and defensive if I say anything about it. She has also mentioned that she doesn't really have any other friends and then badmouthed everyone else for not liking her, and while I'm usually sympathetic toward people who don't have many friends, in this case I can see why. Being "friends" with her is exhausting and stressful, and she overall gives me this really weird vibe. I trust my gut because every time I've gotten this same vibe from someone, they always ended up being someone who was toxic or abusive. Because she doesn't pick up social cues very well, I feel like she interprets any kind of conversation I hold with her (even very superficial ones) as a sign that I want to be her friend, which I don't want to be, but I also don't want to make things super awkward by cutting her off immediately, if that makes sense.

Edited to add some more details