Okay, sooo, here I go trying to make a genderbend work.
This will be ESPECIALLY hard considering the time Ciel was from.
First
Title
Look at me! I'm informative♪ According to this, Celia would be know as either (The) Honourable Celia Phantomhive OR (The) Lady Celia Phantomhive. It's very tricky as to weather or not she'd get a 'married' title (Countess) since women held very little rights (Even if a queen WAS ruling at the time.) I would say due to circumstances she may have been able to pick up on her Father/Mother's job despite how Victorian woman were suppose to seem weak and innocent. Or as my friend pointed out, it can go like this:
Fwend: This be confusing.
meh: I know
meh: we in territory we does not know
Fwend: ...I've got it. It can be an honorary bestowed title from having impressed the Queen.
meh: Impressed her with whut?
Fwend: On behalf of outstanding services rendered in protecting the Kingdom.
meh: ...when did a 12 yrs old do this?
Fwend: IDK.... She said she wanted to prove herself and the Queen in her lulz said "do this impossible thing and I'll let you continue daddeh's work" and so she turned to Sebby "du dis. It's an ORDER." "Yes, Mistress~"
Because she's badass like that.
Anyway, lets go to historyyyyyy~
History
Growing up, one could assume Ciel himself was rather innocent, slowly being exposed to what he needed to do to take on the burden his father carried. Elizabeth described their house as a house of laughter before his parents were assassinated and he himself kidnapped and tossed around as a slave on the black market.
I would have to say Celia would have been raised similarly, probably more lady like but able to still defend herself ( or at least try since she's a child.) Her father being the queen's watchdog would probably want the best possible future for her, to keep her knowledgeable but still seem like a delicate flower until she needed to show her thorns.
According the the manga Ciel was kidnapped at the age of 10 (on his birthday) and put into the black market. He was bought and branded with the mark of the noble beast. It appears a lot of time had elapsed (around a month), and the poor boy endured a lot of physical and probably emotional abuse (rape was insinuated but not confirmed, but do to his reaction of people touching him and the bruise marks on his body I have high suspicions) . Finally after what appears to be a month goes by he's used as a sacrifice, placed upon a slab and stabbed in order to summon a demon. Unfortunately for the people around him, the demon chose him as his master seeing as he was the one who gave up something (his life) in order to summon him. Ciel agreed without hesitation saying he wanted power to take revenge on the ones who did this to us, us being his family that was murdered in cold blood.
Once they agreed to make a contract, the demon placed a seal on his right eye. After that, Ciel made three requests.
That the demon protected him and never betrays him until he gets his revenge, he obeys his orders unconditionally, and that he never tells a lie (to his master).
Upon that agreement, the demon killed everyone that was there at the party and burned the house down.
During the spree, Ciel was able to get what appears to be his father's ring from the carnage. He declares he won't lose like his predecessor had before him. completely determined to get revenge in any way he can. The demon seems amused by how serious the boy is, of course Ciel doesn't appreciate his humor one bit. The demon offers to take him to the manor but Ciel says he doesn't know where it is, much less where they are at the moment. What he DOES know is that his Aunt works at the royal hospital and asks the demon to take him there. Ciel then asks the demon for his name, which he replies with "Whatever my master wishes". He promptly names him after their dog, Sebastian.
They made their way to the hospital, Ciel had inquired at the front desk about his aunt before his old butler recognized him. After talking with the former butler Ciel found out the older gentleman didn't know who had attacked him. His Aunt was relieved to see him, asking him to stay with her for a little until he was better. The boy politely declined saying he wanted to see the house, his aunt then gave him another family ring, telling him it was the only one that survived the fire. Taking it, he then headed to the manor.
Looking at the ruins of his house, the shock of what really happened seemed to hit him. He moved off to the left, finding the family grave site. He knelt in front of his mother and father's grave, calling them as such for the first and last time. Staying in his emo corner until sunset, Sebastian had fixed up the mansion with his demon magic in the meantime, surprising the boy completely.
Unfortunately fixing up the house was all he could do right. The demon made the bathwater too hot and the food too spicy. Eventually he settled on warm milk which Ciel scolded him after saying he wouldn't live on something like this like a puppy, he HAD to get better at what he did. After a bit of talking they decided they'd help each other achieve the roles they desired. Sebastian helping tutor him and teaching him gentlemanly things to do. (For Celia it'll be a mixed bag of lady-like things and also a few gentlemanly activities. Like learning to shoot a gun, horseriding and sword play .)
Things victorian ladies did They went tit for tat when it came to perfecting each other, not holding back.
There was something that Ciel seemed to suffer from, that being nightmares. Whether or not his house was being attacked by people wanting vengeance on the Phantomhive household he would wake up in a cold sweat either silently or screaming. Sebastian was kind enough to check up on him, the poor boy didn't know where he was usually, just knowing the dead was glaring at him in his dreams.
Eventually they became better in general. Sebastian cooked and made tea to perfection (along with killing intruders silently so his master may sleep through the night) and Ciel made perfect scores in his studies. Soon Ciel received a letter from the Queen, her majesty expressed her deepest regret at what happened to his family and said that she'd like to give back the title that his father received in a special decoration ceremony. Ciel has special clothes made for him and goes to attend it at the set date of March 17th at Buckingham Palace.
If you refer back up at the title, for what I think would happen with Celia, then that would be cool♥
She gets knighted as Countess Celia Phantomhive.
When getting knighted, Madam Red (Ciel's Aunt) Walked up behind Sebastian asking why he didn't move closer since he was as good as family, Sebastian replied that would be presumptuous since he's just 'one hell of a butler' (Oh the puns)
After the ceremony Sebby teases Ciel that with the new title he can get anything now, Ciel agrees but he restates that he wants to move forward and get revenge on the people who did this to his family.
Eventually they get a maid, a cook and a gardener, all how have different roles if there is ever an attack on the Phantomhive Manor. Their loyalty is completely and utterly to Ciel who gave them a second chance at life. Ciel also in this time starts up a toy company called Funtom.
Within this time, Ciel becomes notoriously known as a game king, you have a game, he'll most likely excel and beat it easily games are also what he plays when trying to catch people who are out to harm the English people and the Queen, he is the Queen's Watchdog.
Eventually the manor settles into being a dysfunctional family in all the right ways. For my gender swapped version, the only other gender I'm going to swap will be Lizzy, Ciel's fiance because she does play an important part waaaay later on in the story.
So Lizzy visits, causes a commotion, and leaves. Ciel starts playing a dangerous game of cat and mouse and ends up getting kidnapped and beaten by the italian mafia, Sebby quickly comes to his rescue and demonstrates exactly what he's capable of. Ciel declares right before Sebastian finishes the leader off 'Game Over' in a rather cold fashion.
Life goes on and he does charity and then suddenly he gets a mission letter and heads to London during the Social season (summer, really) and his house there Madam Red and Lau greet him before they start discussing the case he received which happens to be about Jack the Ripper. And that's where I'm going to be taking her from in the story, her background will mirror his or at least have similarities.