My cat does not like to interact with people at all! What can I do?

I took a young adult female cat (not much over a kitten really) as a favour to a friend in February last year. Her name is Marmite and she is a healthy beautiful cat, she has safe space to play outdoors and lots of nice warm snuggly places to sit inside, she gets fed regularly (with food she likes, and will happily "ask" me for food) and for the most part seems to be a happy cat. She knows that if she miaows at me I will open doors for her etc and I think that she has accepted me as a housemate.

The only issue is in the entire time I have had her she has not once come to me for affection and she bolts whenever anyone walks too near to her or goes to stroke her.

Since she moved in she has grown immensely in confidence. She has gone from lurking in the corner and hiding behind the washing machine all the time to openly walking into the room when I am there, playing furiously with her toys, and openly getting me out of bed on weekends when she feels I have had enough sleep and she is hungry.

However I have not stroked her once in all this time and she runs away if I get too near to her (if she is in her cat bed she usually just gives me a dirty look and sometimes hisses if she feels like I am invading her space).

Is there anything else I can do to Help her feel more confident or is it just a case of some cats are just not that friendly or take a long time to get used to people?

I am not considering getting rid of her in favour of an instant joy kitten, if she is just an anti social cat she is still my Marmite and I will just have to get used to her I suppose...would getting an additional cat help do you think?

Cheers