I've always subscribed to the theory that best friends are the ones that laugh at you (which quickly turns into 'with you') and call you uncomplimentary names (to which you immediately respond in kind) but if anyone else tries it, they will be at your defense in moments.
As far as I'm concerned, Masahiro and Guren from Shonen Onmyouji have one of the best friendships that I have ever seen on an anime show. Probably one of the best friendships that I have ever seen anywhere.
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