He's a friend of mine
The message is clear. He's my friend. He wishes all good for me and helps me whenever I need one.
He's an enemy of mine
The message is clear again. He's my enemy. He wishes all bad for me and never helps me whenever I need!
He's ...... of mine!
The message is clear. He's neither friend nor enemy. He's in between. At times, he may help me but then I cannot always rely on him. He's not a friend of mine.
Other times, he does really bad to me but maybe, there was no any other go for him. He's not an enemy of mine! :)