<human>: Summarize the given text in a few sentences.

I'm a reasonably attractive female, and my best friend is a reasonably attractive straight male. We have never dated. 

I was briefly attracted to him at the beginning of our friendship, but the more I got to know him, the better I liked him and the less I was attracted to him. I believe the feeling is mutual.

I know "best friend" is a childish title, but he means a lot to me and we understand each other better than most people. He is a little fucked up when it comes to romantic relationships, calculating and distant from what I have been told and observed, but he's a great friend. I have a lot of anxiety issues, to which he is sensitive, and he has helped me come out of my shell a lot since we became friends. 

We historically have befriended each other's S/Os, which allows us to all hang out together, but it's not going to happen here.

He's had a flirtation with a girl for a long time, about 2.5 years, and it seems to be coming to a head.  She has been rather doggedly "chasing" him and trying to get him to settle down for a long time. He has formally dated 2 other girls during this time and hooked up with a lot of other ones, and for this reason, I guess she sees all other girls as a threat, including me.

About a year ago, she confronted me and told me I was "sad and desperate" for "clinging" to him, despite the fact that I was in a committed relationship at that time (I am not currently). She seems to think I am secretly holding a torch for him, and, according to mutual friends, tells him that I am jealous of her for "being able to do what [I] never could".

I honestly don't know what to do. He seems to really like her, which is rare for him. I feel like if I tell him that she's being mean to me or ask not to have her around when we hang out, he might think I'm trying to sabotage their relationship and keep him to myself. I don't want to hang out with someone who hates me and I don't know whether I can confront her or anything.
<bot>: My best friend is about to start dating a girl who sees me as a romantic threat (which I am not and do not want to be) and I don't know what to do.