Fimbl

A friend of yours, that you simultaneously love and hate.

A hated friend of yours: "Am I your best friend?"
You: "Hell no, you're my fimbl"

Frumbl

A good friend, that excels every other friend of yours, but still doesn't quite reach your best friend's status.

A good friend of yours: "Am I your best friend?"
You: "No, but you're my frumbl"