‘Rather’ here has the meaning of ‘somewhat’.
That is, to some extent or degree:
She’s been behaving rather strangely. – She’s been behaving somewhat strangely / She has been behaving strangely to some extent.
He’s rather an unpleasant man. – He’s a somewhat unpleasant man / He is unpleasant to some extent.
She rather turned against me. – She turned against me somewhat / She turned against me to some extent.