Heroism
Yuuhi is in every way the intrepid hero of the story. Personally I view Tooya as little more than the 'lover' in Ayashi no Ceres, though his role in the story expands towards the end, his character is ultimately as Aya's partner. That's not to disregard Yuuhi's fair share of romancing with the main character and maybe even his own love interest, but love interest or no, it never stands to fail that Yuuhi is the one that comes to the rescue. Whether it's Aya, Suzumi, Chidori, whoever... if someone is in need of rescuing, Yuuhi will step in prepared to do whatever it takes to help [including beating people silly with metal chopsticks]. Though he maybe reluctant on occasion, his heart and pride win over his cowardliness anyday.
Though admittedly he's not always doing the heroism, having a strong female lead with her own supernatural powers helps, not to mention where would a love story go with one of the partners constantly being saved by another man? But if there's one thing I associate with Yuuhi it's his willingness to help others; especially those he loves.
Although it may seem as though i'm going by archetypes alone in this analysis, one has to remember that Yuuhi is a character. A beautifully created, depth filled, fictional being. And one that, for me personally, can both inspire and enrapt you with his steady heart and prevailing sweetness.