Of course these usages overlap, and change with context, it would be wrong to be too definitive about it. However, in the context of friendship, the term "buddy" has an informality that often, I feel, reflects a particular warmth and spontaneity in a relationship. So someone who is described or seen as a close/best/bosom buddy would be a special friend. It's common in reference to kids: "Jimmy's made a lot of new friends at school and, luckily, his best buddy lives just around the corner". "Best mate" would be more common here, but I hear "best buddy" all the time.
not a teacher