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I think we have the kind of friendship were if I were the devil, you’d be the only one I would tell.
— Aaron · Broadcast News

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
A wry way of saying I trust you completely. The speaker imagines the most extreme secret possible, being the devil, and says this friend would be the only one told. Friendship is measured by how dark a confession it could hold.
Context reviewed