I am friends with someone who is nice to me but my friend group doesn't like them?

It’s difficult to give you thoughts, because we don’t know the details about why the group ostracized Z. We don’t know if they acted unfairly.

You have a very delicate balancing act to attempt. I wish you luck.

It isn’t easy to maintain2 friend groups in school, but it can be done.

I would be loyal to the one who acts like a friend to me, and not to the ones who cast people out. Assuming, as @Hawaii_Jake says, that they didn’t have real cause for it.

What if you stuck to Z, and other people who admired your friendship were interested in joining you? Then you would have a new friend group.

You wrote that Z is kind and calm with you.

Does that mean that Z is not kind or calm with your friend group?

Do your own thinking, don’t follow the crowd. The crowd is often full of crap as a holiday turkey.

Your friends are people you trust and whose opinions you respect. Have you asked yourself why your circle of friends has so vehemently rejected Z? You might also consider why Z has no friends except for you.

If Z is truly as kind and good as you’ve written, continue your relationship with her. You’ll just need to keep that friendship separate and apart from your friend group.

@Love_my_doggie makes a good point.