I was looking to express a concept recently and realised I didn't have the terminology for it, so I decided to coin something.
So I now have a new category if friendship:
A 'real' friend is someone who I can express an opinion to, disagree with and still be friends with.
This is mostly independent of whether they are also a 'good' friend (someone with whom there is a lot of shared knowledge and time) or a 'true' friend (stuck with the friendship through crap, or at least given me strong reason to believe they would even if they haven't had to yet).
Of course being in any of the categories makes moving into the others more likely…