Been while since i read that. Did Yoriko and Yuu really going out ? If my memomries were right, Yuu is more of a free spirit and is not really fixed on someone. It feel really sad for Sonoa.
Edit : Reread a bit and well it seem Yuu love Yoriko but feel to me like it's one sided. Yuu, i know you're in debt for Yuriko but you can't break Sonoa's heart like that. That's why i hate love triangle, it always let a girl on the side, at least in the MC triangle if what i think about Yuu/Yoriko/Sonoa is true.