I want this girl. She is single, but her best friend and her are seemingly obsessed with eachother and I'm told they have casually had sex. And before you ask, yes, I am very certain they are not together. At events I go to she is always with him, and her best friend does not like me. If I approach her, he will see me off. I have game and I know if I could approach her semi-privately I would do well, however it is difficult with this man always with her.

I'm not PUA but I can see the problem. Of course he doesn't like you, if he has slept with her then he basically considers her his territory so obviously he's not going to like that someone is trying to chat her up.
I for example am guilty of stuff like this, even with girls I haven't hooked up with but I know like me and I have no interest, it bugs the shit out of me when someone is trying to chat them up because my primal instinct tells me they are "mine".
Instead of going to her directly try to open to him get some common ground, win him over and he wont be such an obstacle for you later on.

I would agree, but I have not made any advances towards her yet. He naturally just dislikes me, I know not the reason.

So, does this change anything?

The reason is you are invading his territory, like I said try to try to find some common ground with him,lower his shield! If you see him with her talk to both of them not just her as he may feel alienated from the group, if you try to be his friend and he acts like a dick it will DHV you and DLV him because your the nice social guy trying to be everyone's friend and he's just a jelous douche hope that sort of helps!

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Suriyel - According to the Book of Enoch, Suriyel was one of the leaders of angels who lusted after the daughters of men.

Alright man, thanks.