Towa (uke-omega), Seiji (alpha), Aiji (omega) -
When they were young, Aiji went into heat and Seiji bit him, them their parents thought about sending the omega away and seiji felt guilty. seiji resisted, parents gave up, they moved out and lived together. Both of them try to copy each other so the other doesn't have to be discriminated (since seiji is guilty and aiji is 'angry' it turns out that aiji was sort of manipulating him but he, aiji, does realize it). later on stuff happened, they meet uke, kept him as a "pet" <-- nicely!, discovered that it was aiji's plan to find seiji an actual mate.
Ai to Makoto (Moriyo)