Shuro
I suppose Yuuhi's interactions with Shuro are something akin to a buddy buddy male relationship. This is mostly due to the fact that Yuuhi doesn't actually realise that Shuro is a girl. And Shuro, having been brought up forced into being a male, didn't act as though she weren't. As far as his non-romantic interactions go, I love their relationship. We get to see Yuuhi acting like a normal guy instead of having to constantly guard or mope after Aya. I love how Shuro exploits the fact that Yuuhi doesn't realise she's a girl to the fullest, comforting him like a girl would and even giving him a kiss on the cheek, making him flip out in one of their many humorous scenes together.