Use cordial to describe a relationship that is friendly and sincere but not overly close. A cordial greeting is perfectly pleasant but doesn't involve lots of hugging or excessive emotion.
You might have cordial relationships with most of the kids in your close, but only confide in your closest friends. In Middle English, this adjective meant "of the heart," borrowed from medieval Latin cordiālis, from Latin cor, "heart." This core sense of "heart" can be seen in the synonyms heartfelt and hearty. The noun cordial originally referred to a medicine or drink that stimulates the heart, but its current sense is "a liqueur."