byakuya omega

when byakuya presents as an omega as a teenager, suddenly everything makes sense. his talent for magic was so much lower than his brother kariya's, because properly formed magic circuits interfere with omega male biology. something about the two systems do not play nicely with each other, so they're seen in magus families as bad luck because they aren't capable heirs. byakuya is locked away once he goes into his first heat and doesn't come out much after due to embarrassment of his nature. byakuya is a pretty smart kid despite his bad attitude, so he spends a lot of time reading as an escape from his home life. zouken gives him shit constantly for being omega and kariya never presents, so byakuya is jealous of him for being both "normal" and capable of magic. their relationship tanks and kariya denounces his family and leaves as soon as he can.

because the potential heir is gone, zouken has to turn to byakuya for other options. there are alphas from mid-level magus families that offer their "services" to other families in need of children with healthy magic circuits. byakuya is very against this, but with this alpha introduced to him in the throes of a heat, he doesn't have the self control to hold himself back and they remain together for several days until its conclusion. within a few agonizingly slow and nervous weeks, the process is confirmed a success. byakuya low-key craves death.

byakuya is left to his own devices for the duration, which is both a blessing and his own personal hell. it isn't until kariya reluctantly returns home to check on things after byakuya refuses to answer his calls that he (begrudgingly) gets some help.

a male omega has an equal chance of producing a female or male child, but if that child is male he will almost always present omega. there is a slight chance he will be beta and virtually no chance he will present as an alpha. this makes byakuya's condition all the more stressful because even after everything, the likelihood of producing a suitable heir is still cut down from that of a beta or alpha. what's more, the sex of the child isn't known for quite some time and even after the child is born there is still a while before the strength of their magic circuits can be determined. the stress of this leaves byakuya sick and miserable most of the time, often blaming kariya when his emotions get the better of him.

the child is a boy. his name is shinji. byakuya is worried. kariya is worried. the three of them quietly play house while a collection of horrific possibilities loom over them. still, there is genuine affection and a strong desire to protect shinji. when their worst fear is confirmed and his circuits are deemed malformed, byakuya is punished. despite this, byakuya considers that it might be better for shinji to not have to endure the family's magic crest. having given up on his own bloodline, zouken outsources to the tohsaka family, and eventually this sparks a whole new bout of rage from kariya, who can no longer handle the guilt from what has come of him refusing to be the family head.

shinji is just starting school as all of this is happening, where he's excelling naturally both socially and academically. though, noting his family structure is glaringly different than those of his friends at school he starts to ask questions. with second natured people being mostly common in families with magical lineage, he doesn't hear of anything similar to his situation. rather than telling shinji the truth at an early age, byakuya succumbs to the stress of years of being taught to feel ashamed of his nature and lies to him about his mother having left them both behind. as talks of the fourth war are just beginning, byakuya decides to hide shinji from the world of magic all together and sends him abroad to russia for schooling. as shinji leaves, sakura tohsaka arrives and kariya begins his training. byakuya drinks to cope.

when kiritsugu emiya breaks into the house in search of irisviel, byakuya is so inebriated that he hardly remembers what happened to this day. in a fit of adrenaline and rage, kiritsugu can't help but catch byakuya's frightened and stressed omega scent and the feeling that twists in his gut from it pulls the trigger that shoots his hand and leg. kiritsugu leaves in a rush and byakuya lies there with no one to help him until he's picked up by search and rescue quite some time later. the amount of time between the injuries and medical help result in amputation of his hand and a lifetime of pain and trouble walking. he spends quite some time in the hospital sullen and doped up on pain meds. kiritsugu, walking by with shirou in his arms notices the familiar scent and sees byakuya in a wheel chair in the hall. he feels a pang of guilt and a weird sense of attraction that won't stop itching at him. the hospital, being inundated with people after the fires, doesn't do much in the way of a background check when kiritsugu claims byakuya as one of his own and takes both him and shirou home.

settling into kiritsugu's house is difficult and byakuya is rigid and untrusting but also not wanting to return home, either. eventually, he begins to realize what an actual dope kiritsugu is and begins to lighten up a little. recovering in the hospital was a crash course in rehab from alcoholism so he's a little shaken and erratic now and then, but kiritsugu lets him  bum a cigarette or three every night if he joins him out on the veranda and they slowly begin to bond through his recovery. even shirou is helpful, learning to cook meals and keeping schedule of byakuya's  pain medications. in a few months time the three of them fall into a relatively easygoing style of daily life. kiritsugu's second nature becomes obvious very early to byakuya and isn't long at all before they're both very comfortable and seeing to each other's needs both physically and emotionally. it's something byakuya has never had before and he often finds himself perplexed by it.

after shinji has been in school overseas for three years his pre-paid tuition has run out and obviously, byakuya has no more money for shinji to continue to attend. realizing he has never mentioned to kiritsugu that he has a child of his own, because it would bring up a lot of questions he doesn't particularly want to answer, he tries to pass it off very nonchalantly and it works about as well as you would expect. kiritsugu takes it pretty hard, feeling guilt from having physically crippled byakuya when he has a child to support. in an attempt to redeem himself, he eagerly accepts shinji into his home. it is very obvious that shinji is byakuya's biological child, looking like a miniature version of him. he's not very happy about being uprooted from his school and his friends, however, and his adjustment period is rough. he refuses to speak japanese, while byakuya hasn't retained enough russian to have a proper conversation and he is constantly shocked over being "poor", having been in a high society boarding school for so long. shirou is persistently friendly with him, however, and eventually wears his prickly disposition down. they become friends and shinji begins to soften up around the adults as well.

the closer that shinji and shirou become, however, the more concerned byakuya starts to feel. while second natured children don't typically present until their mid-to-late teenage years, there are hints if you know what to look for. with shinji having such a high chance of presenting as an omega, byakuya is already worried about him and it causes him to realize that if shirou is alpha (which he has seen hints of) it could be a big problem. once these fears about both children are essentially confirmed, byakuya is in full stress and panic mode and kiritsugu has to try to decide how to proceed with all of them. it would be cruel to keep them away from each other when they obviously give one another so much comfort, but if they get too attached too early they might make mistakes while they are still way too young. the two of them wonder how to educate their children on second natured and reeling in impulses while shirou and shinji sneak around to act on said impulses.