OK, here is a description of my OCs from my post-HF universe. I'm going to start outlining the basic setting, then I'll give a description of Sakura's four children, and also Kariya's girlfriend/wife. Finally, I'm going to put what I have so far of Aoi's character sheet in another post, which I could do with some help fleshing-out (in particular, her personality I am somewhat unsure about, and some ideas would be good).
Setting
These characters all come from a post-HF True universe. Sakura and Shirou are happily-married and have four children. They still live in the Emiya house with their youngest child, Kiyoshi and Rider. Their two eldest childen, Aoi and Kariya, have both married and moved out, with Aoi now living with her husband in a nearby house purchased for her by Sakura, and Kariya living wherever he works (which will vary depending on when this is set) along with his girlfriend/fiancee/wife Sam.
Sakura works at an orphanage which she also provides much of the funding for, and is also the patron of several children's charities, especially ones related to helping abused children. Whilst her nature as a magus prevents her fostering orphans as she would like to, she does care deeply for them, and does sometimes invite them over to her house to meet her family and eat a delicious home-cooked meal. She also does cook for them at the orphanage (although less often than she would like, due to her own family commitments), bringing Kiyoshi over to meet the orphans. She sees them as almost part of her family, and is known to be rather generous towards them, both whilst at the orphanage and afterwards.
Due to the properties and other wealth which Zouken amassed whilst alive, and which passed to Sakura as his only remaining "descendant" on his death, plus her own intelligent investment of those resources, Sakura is wealthy and has no need to work, and nor do her children if they do not desire to do so. Despite this, Shirou in particular encourages them to go off and make their own way in life, rather than sponging off their mother's generousity. Of course, he has no objection to providing anything they need, regardless of expense.
Since Sakura has no inherited magic crest of her own, she has created a basic one to pass onto each of her children. This allows them to use her magic without learning it, much like Rin can with the Tohsaka magic. However, rather than Sakura's altered affinity to binding magic, they have inherited her natural affinity to the Tohsaka magecraft, transfer of energy, and thus are capable of being jewel mages like their aunt.
All of the children are relatively disrespectful towards their parents (especially Sakura), but in an affectionate and loving way. She is generally very soft on them and spoils them to a large extent, due to how horrible her own upbringing was. However, she's not incapable of telling them off or ordering them around, although she will only do so when they are in real danger or when they are being completely unreasonable. There are a few things that will set her off, though, in particular misuse of their magic (due to her past experiences as Dark Sakura), and also them putting theirselves in unnecessary danger or treating others (especially those who at first glance appear to be "dangerous" or unusual) badly without just cause.
Characters
Emiya Aoi:
Eldest child of Shirou and Sakura, Aoi was born with exceptional magical talent, almost on the level of her mother and Aunt. As a result, Rin encouraged her to take the path of a magus. She considered the possibility of handing over the Tohsaka magic crest to Aoi, but at the time Rin was still doing her own research in London, and didn't really have the time or inclination to take on an heir, particularly since Sakura was unwilling to let Rin take her daughter away to London, at least until she was old enough to make the decision for herself.
However, whilst Rin encouraged her to follow the path of a magus to some extent, her parents encouraged her to consider other pursuits as well, and to live as normal a life as possible, although they didn't discourage her from learning magic. As a result, whilst she has decided to actively pursue being a magus, she hasn’t lost touch of her human side. She also remained at home, since her pedigree as a magus wasn’t good enough to get a place at the Association, plus she didn’t really want to leave her mother.
Has developed several spells that combine her shadow affinity with her jewel magic and, influenced by her brother Kariya and his girlfriend, named them "Supernova" and "Black hole". Probably had an accident at some point when practicing her black hole spell, very nearly swallowing up her entire family, until Sakura stopped it, nearly dying in the process. After that Sakura ensured she was a lot more careful with her experimentation, and also made her build in some safeguards to the spell to ensure she wouldn’t be able to cause serious damage with it.
Sometimes acts as the Second Owner of Fuyuki if Rin and Sakura are both absent, and also assists them in performing their duties when necessary.
Lives with her husband in a house close to the Emiya house, purchased for her by her mother. Shares Sakura's affinity for the shadow magic, and thus is more capable of manipulating her shadow dimension than any of her siblings are. The strongest magus of the Emiya children, and the only one capable of putting things into the shadow dimension safely and without corrupting them (particularly people).
Emiya Kariya:
Second child of Shirou and Sakura. Born with fewer magic circuits than most of his siblings, and with little ability to manipulate the shadow (can, however, act as an exit point for people inside the shadow dimension, with his mother's help). Never really took to the magic training, and instead of becoming a magus, went to university.
Married (or at least engaged) to Sam, after meeting her through an orphaned friend who Sakura had funded to study physics at the university. Where he lives depends on the timeline, as does his marital status.
Tohsaka Yuki:
She is the third eldest child of Shirou and Sakura. Implanted with Rin's magic crest at an early age (or, at very least, began preparations to do so), after Rin had asked Sakura if she could adopt one of her children as an heir. Even though Sakura trusts Rin, she had considered the situation to be too much like her own, and could not bring herself to abandon her child, even to what would surely be a very loving mother. However, as a peace offering, she had agreed to allow Rin to train the child as if it were her own. Formally adopted by Rin at the age of ~12, after asking to live with her to aid her training Extremely powerful magus, possessing both the Tohsaka magic crest, and also the simple one that Sakura had produced for all her children.
Emiya Kiyoshi:
Fourth child of Sakura and Shirou, and the only one Sakura still actively looks after. Due to his young age relative to the others, Sakura adores him and is highly protective of him (even more so than of her other children), doubly-so since he looks rather like a younger version of his father, although he does have his mother's natural green eyes. In addition, Sakura tends to spoil him somewhat (as she does all of her children), although she gets rather upset at him if he takes his life for granted.
Like his brother Kariya, he is only an average magus. However, unlike Kariya he was determined from a young age to make the most of the limited talents that he had, and use them to fight evil. Initially, he seemed entirely hopeless, but he found that he was capable of manipulating the ether clumps he created with his failed efforts into useful items, and his Aunt later confirmed his elemental affinity. He also found himself to be good at reinforcement magic, and thus uses this regularly.
A reasonably powerful magus, and one who seems to be picking up on his father's former ideal and self-sacrificial nature to some extent. Sakura finds his fixation on Shirou's ideal somewhat cute and endearing, although she does worry that he will come to harm as a result. Is learning to fight using shadow swords created from Ether with some shadow magic imbued into them, but is as-of-yet not anything like powerful enough to take on his father or mother in a battle. However, anyone who goes up against him should be wary that, if they hurt him in any way, Sakura will be extremely upset and angry.
Nice and caring, but sometimes unintentially selfish. Doesn't realise when his actions are harming people due to being somewhat sheltered. Additionally, due to the sheltered nature of his upbringing, he is somewhat reliant on his parents (Sakura in particular) taking care of him, and does not feel comfortable being away from them in unusual situations. When fighting, he is somewhat hot-headed, often treating conflict as the first option rather than a last resort. He also genuinely enjoys beating up bad guys, although he does prefer not to kill when possible. Also very poor at doing things he doesn't enjoy or want to do, particularly household chores (due to his parents tending to do them). However, he does enjoy cooking, as a result of seeing his parents' enjoyment of it and having had lessons which they also made enjoyable.
He enjoy sword-fighting with his father and Rider, using low-power versions of his shadow swords which will not cause any real damage to Shirou if they hit (Shirou fights using swords which will shatter on hitting Kiyoshi's body). Also enjoys play-fighting with Rider (although she does not fight anything like seriously) and trains in magic with his mother, as well as his big sister Aoi and Aunt Rin on occasion. However, he has no time for the principles behind magic (much to the annoyance of his aunt), instead only caring about the aspects which he can use to increase his own fighting prowess.
Tends not to listen to Sakura much, because she is extremely soft on him. However, he does still love her and trust her to protect him even though he tends not to obey her, and he is old and smart enough to know when he really should listen to her, for his own safety. Particularly since she can actually be very scary and authoritative when she is genuinely concerned for his well-being as opposed to just trying to get him to do something.
His Aunt and sister Aoi are trying to teach him to use magecraft more effectively, in particular with regards to use of the family jewel magecraft, but he shows little interest in their lessons. He also lacks the ability to do conventional magecraft well, although he is capable of the basics.
Samantha Emiya (Sam):
Very smart quantum physicist who moved to Japan to take up a postdoc place, and then met and (eventually) married Kariya, through a mutual friend that Sakura had funded through university after his parents "mysteriously" disappeared without a trace when he was very young. An expert in Many-Worlds theory of QM and, until her experiments went wrong, was close to finding a method of replicating the Second Magic using science.
Has discussed the Second Magic with Rin, and was amazed at her ability to do things which she spends her entire life studying so casually. Also explains to Rin how Many-Worlds works, and how alternate dimensions work.