extraplatonic

A platonic relationship with a close friend with whom you share everything, even more than you would with a romantic partner.

Guy1: So, what's between you and Em, huh?
Guy2: We're just mates, man.
G1: What do you mean?! You're just friends?! But you guys talk about anything under the sun!
G2: Of course we're just friends, we haven't ever been any more than extraplatonic. I love her to bits though, she's the best.