Tooya
Yuuhi spends very little time around guys. In fact, the only constant male interaction we really see is between him and Tooya, and instead of getting to see some friendly boy action, we get tense moments and an explicit feeling of rivalry. Because they both want the same role: protecting Aya. However they do realise that Aya needs them both, and grudgingly accept the others importance. Well, I say grudgingly, it's more on Yuuhi's part than Tooyas [Tooya's got the girl after all]. However I do have a feeling that Tooya may have envied Yuuhi somewhat, because he was self assured and knew what he wanted.
Although Yuuhi didn't grow up in an easy atmosphere, he had people who cared for him, a memory to look back on. In short: he knows who he is. Tooya on the other hand doesn't uncover his past until the very end of the series. Up until that point he has many doubts and fears pertaining to his past and who he may have been. These insecurities plagued the beginning of his relationship with Aya, and his ultimate fear of hurting her. When I say Tooya envied Yuuhi this is just something of an assumption, but it's obvious that there was an appreciation for the solidity that Yuuhi presented. If Tooya failed as a person and lover, Aya had a good steady man to fall back on.
This is confirmed when they go to rescue Chidori and Shouta: Tooya thanks Yuuhi because he had been there for Aya more than he had. We further see the trust between the two when Tooya, at the end of the series, 'asks' Yuuhi to take care of Aya when he dies.