Family
After his mother leaves him, an event that's effect on Yuuhi is painfully apparent if only for his claim that his mother had died, Yuuhi becomes estranged from the family. His stepmother was horrible to him, and he felt he hated or was hated by his father and second elder brother. The one person who was there for him was his eldest brother Kazama, who, when he married Suzumi, asked that Yuuhi come live with them. Suzumi happily consented and thus started what seemed to be the perfect family...until Kazama died in an accident one year into the marriage. Yuuhi's one true father figure was ripped away from him but no one took it worse than Suzumi. Which is probably the only reason Yuuhi was able to handle it, because Suzumi, the one person left in his world, needed him.
Afterwards the two live together, and when Aya enters the picture Yuuhi's 'family' continues to grow. However he still felt a deep loneliness because of his blood relatives and expresses so to Aya [in a rare moment where he actually opens up to her]. When Suzumi is captured at his fathers house he instantly goes to save her, and finally peace is made with his family. He finds out that his father loved him the most, and that his brother [though a total git] wasn't all that bad either. Even though he continues to live with Suzumi I think he no longer feels the bitterness he once did, and realises his family is there if he needs them.