Credit goes to @symyne for the name and appearance assistance, kudos to you!
So my own OC is gonna be a fankid because that is what I love to do, daughter of Ferdinand who looks nothing like him other than the shape of his face and his perpetually furrowed brows. She has white as snow hair and crimson blood eyes, her name is Athena and was the offspring of a woman who was kept in secret by Ferdinand himself, as he chose to not utter a single word about her, the woman's name will keep silent for the whole series, not giving a name to her, but she died giving birth to Athena and the girl took her looks, and her appearance would cause a scandal if Ferdinand was open about him having a child looking differently to him or the story of a mysterious woman who would never be even brought up. But Ferdinand trusted Fran with the secret, so only him before the settings of the main story knows of Athena's existence.
How would I describe Athena? Being raised by Ferdinand himself in the dark, I would say he did raise her properly without looking himself like that harpy of Vero and he never really hit her or verbally abused her, but still was quite strict with her(insert here a ChiChi from DBZ level of strictness) so the girl is utterly polite, can go serious, but at some point she starts smiling and relaxing herself around others when she is brought out of the darkness(not really a carbon copy of Ferdinand, for one, it would not be original and considered copy paste copyright stuff and for two, I like to give my liberties to my fankids), she has a dark obsession with dark science, like she thinks matriphagy is quite interested, creeps the living crap out of her father when she goes suddenly asking "Father, if I don't have a mother does it mean I ate her up when I was an infant?" which prompts Ferdinand to nip in the bud her tomfoolery(by writing lines, he's never hit her, past a tiny forehead flick, not going violent on his own kid) but she's being once or twice in the Repentance Room, because unlike Myne/Rozemyne she is not as frail as she is, in fact she is quite tough for her age, and usually is just half an hour for her because she's shown to learn her lessons after being asked why she was sent there to ponder on her ways and actions. Never has cried like other children, as a baby obviously she did because no baby can be too serious to never cry and Ferdinand had told her that she is allowed to cry when needed, but she chose not to, because in nobility a crying child is the same as a spoiled brat, so Atheny chooses not to go through the hassel of shedding tears at all, she is very lady like and prefers sitting elegantly watching Papa brew potions, see him work with magic and stuff. And when she's sick, her own doctor is her own dad because he also can be plenty of things at once, her tutor, her teacher, her father, her doctor and even his disciplinarian, what no, and is cool. Another thing, Athena's personality is a mix from own dad's personality and from Hiiragi, another girl from another anime I used to watch a decade ago(Hanamaru Kindergarten). So she is quiet, serious, dark science obsessed, smiley at times and very sassy(you would think a sassy kid would never live with Ferdinand as their father, but she lives, only her hands stings because for every sassy response she utters, it's another hundred of lines, safe from abuse/verbal and emotional, but not from strict parenting.)
When she meets Myne/Rozemyne, she doesn't think much about her, just a simple commoner that came to the Cathedral where her dad serves as a High Priest. And when Myne/Rozemyne started having interactions with Ferdinand, the true identity of Athena was not disclosed until Myne/Rozemyne discovered it herself prior to her meeting Silvester and Ferdinand told her the truth of his daughter and how she should keep it a secret too. Now not only Fran knows, now the bookworm does too and she does keep it, she believes Athena would be like other kids, but the child has surprised even her adult brain, when she would not play with the Shumil plushies she brought her once, the girl had stated those things stress her out, she has stated also she is not her average girl to be invested into tomfoolery, basically sasses her too many times and there will be times Myne/Rozemyne would snap at Athena out of sheer irritation(just like how she almost obliterate bratty Wilfred when he started being a brat to her) and then Ferdinand must come and yanked off both girls by their scruffs like naughty cats, for Athena he gives her ten hundred lines to write and many potion names to be memorized before dusk and for Myne/Rozemyne cheek pinches and lessons to keep her mana in control. So in other words, Ferdinand is stuck with not just one gremlin but two of them. Thankfully though, Athena backs down but is in not very good terms with Myne/Rozemyne. That happens in Part one and Part two. They start to get along, when the events of Part 3 start to happen.
Oh she absolutely can't stand Wilfred. Gives him the sharp tongue of Papa because boy must learn the hard way not to be such a brat. Gets along though, after the family becomes a real family and not a political thing.
Silvester meets his hidden niece and starts pinching her cheek like how he did the first time he met Myne/Rozemyne. Athena was close to biting him, but her father tackled her immediately and flicked her forehead. No child can be a feral tiny beast. Silvester learns not to poke the feral cat or else he looses a finger.
But to keep Athena safe they also craft a fake backstory just like how they did with Myne/Rozemyne and the outside world does not know her as Ferdinand's daughter, but as his long lost recovered niece from a very, very distant other duchy(it's invented, so I won't really give it a name, not important to the plot of Athena)
As stated before, Athena does not cry but there is a first time she broke down into bawling and sobbing, when she heard they would force her father way and is not letting the happen, it's the very first tantrum she throws before Ferdinand and given it's never happened before, Dad is so, SO lost on what to do.
Since I don't have my colors until the weekend, I made a Picrew of Athena, she would look like this:
(Link to the Picrew: https://picrew.me/en/image_maker/186583/complete?cd=6u7kuOQehM#google_vignette)
That is all I have for my very first Honzuki OC!
Had fun coming up with this while I was almost finishing work.