close acquaintance

A friend of a friend, that you have met, and are on friendly terms with. You do not maintain a friendship with them, but know them. Both parties wish not to do damage to your relation with a close acquaintance, as that damage would reflect on individual relationships with your mutual friend. But due to any number of reasons, such as differences in interests or schedules, you cannot truly become friends with them. Not someone you would seek company of, nor particularly enjoy, but respect and appreciate the bond that they share with your friend.