For all of you guys that only rely on the Comic thats here on Danbooru. Everyone pretty much gets split into "Parties". Your see them if you watch the NicoNico Vids, but i guess you wouldnt watch them if you didnt understand japanese.
Well Ryu-san was paired with Iku in an earlier Touhou/Buront Crossover, the Legend of Gluttony, in TIoYaY, he's paired with Mystia. Not sure what thing has him with Okuu.
Such is the peril of being paired in 2-3 distinct series.