Heero Yuy. Trowa Barton. Well. Ok, I can see this happening a little more than some, a lot les than others. Both are quite quiet, and both seem to understand each other. Heero and Trowa have  good unique friendship. They don't really need to say much to know what's going on, and both are good soldiers. I can see them taking comfort in each other in the war, Trowa causing Heero to become a little more human, and Heero getting Trowa to laugh and talk a bit more, you know? 

It's a case of there's evidence, and there's not. It depends how you watch the series. If you look, you can see Trowa finding Heero's actions amusing, causing slight smiles. But then again, if you look really really close you can probably find him doing that to everyone...

(point. No ,that's not an official picture. I couldn't find any pictures with Trowa and Heero together... it's because they're always paired up with the others. Heero with Duo, and Trowa... well, you'll see)