Oathbreaker

That's tricky, as i don't have a full grasp on all the characters myself, and i thought the TF mechanic would be a good storytelling device to explore them beyond the front they have to show during times of crisis and war, which is the events of the game.

Again spoilers because this is my take on all the characters i managed to survive getting hit.

Norde got canonically hit, isn't normal fire emblem quirky so it's hard to tell what his deal is, but i would have him turn into a normal human that looks like a noble one without blemish or scar, and now that he's in the position where people treat him more normally and not for his heritage, would start missing it, and feel people are treating better, which would annoy him more than if they looked down on him. This would be his "Weakness to peace", it would explore how he see's hardship as normal and peace as not, and let him question what he wishes for the world and his place in it.

Satia i would have turn younger, representing her "Weaknesses to children", she would not be physically weaker, in fact she would gain in strength, but by the end she would look like a loli. This would make her mother act impossible, but it would be pointed out, that the sword outright gave her youth, beauty, and strength, in an effort to make her something that cannot mother or be mother,( and certainly she cannot give birth to children even when she does grow older again.)

Rowan did not get hit, and i don't feel i see the man under the emotional mask he wears well enough to guess, he hides his weakness well.

The dog Jackson? I have no idea. Didn't have in party during fight so got unhit by oathbreaker sword.

Charlotte i feel would be a complicated and visceral one. I would have her injury be canonical starting from the end of act.1 fight. starting from where she got hit. she would outright start turning into wood bit by bit, her body would move like normal and be seen as perfectly healthy, but she would be worried about turning into a tree before she can finish the fight. One morning she would seem fine and the transformation seems to have gone, to the shock of everyone else. Yet she would also have a new bow that she would refuse to explain. That bow is her new body, and she is projecting a human illusory one to wield herself. She is the new oathbreaker, the bow Charlotte, a bow that slays those who wields oathbreakers, giving effective damage, and giving immunity to effective damage to the wielders of oathbreakers. Completing her,"Weakness to revenge." Of course her life as a noble would be over if the truth got out, and she would hand her titles as quickly as possible to her sister,( if she's alive), or whoever else is closest to her. Would start regretting and being mournful after the TF. Especially if her sister would survive, who would endlessly worry about her during her TF.

Charlotte's sister Mariel, didn't get her hit wouldn't survive. If i had to decide one, would start turning invisible, as that would be how she feels compared to her outgoing and more charismatic sister. " Weakness to being unseen."

Titania, Her Greed would have her slowly gain draconian features and several kleptomatic impulses, and be forced to face the reality that she has the same greed filled impulses that she hates the nobility for. "Weakness to her own greed."

The pirate Otis, already got hit, already became the in-universe lusty argonian maid equivalent while keeping her old job." Weakness to easy catgirls", is canonical.

Kara, hard to tell at this point in the story, would have no visible signs of change, to freak everyone else out, and probably Kara the most, who wouldn't visibly change, but would get more paranoid. "Weakness unkown?"

Fylnn, not in party during fight but will speculate- would gain a massive confidence boost and start acting more boisterous, whenever they catch themselves they would apologize and look ashamed. It would eventually be explained, that Reagyn and him used to be separately raised, and during his early childhood was raised by his churlish uncle who was a poor father figure not punching abusive, but neglectful. He only cared that Flynn did the bare minimum and not bother him, before said uncle committed a heinous crime and got executed. "Weakness to family shame." It's not the uncle who's the weakness, it's the fear of shame in front of Reagyn the family member he most cares about. Tellingly he never has "uncharacteristic hotheadedness", away from Reagyn.

Reagyn- would give Pegasus noble girl bird wings, Peacock wings, her hair would turn white and explode and become massive and beautifully combed no matter what. She would look like a creature of awe and beauty. Which she hates, since she's a tomboy, the last thing she would want is to be nothing but an object of beauty that can't fill the obligations of nobility. Making her a foil to Pup in a way."Weakness to Beauty", is still a weakness. Still haven't seen much of her character to subject to change.

Jenny- canonical changing blade demoness TF, comes with a class change. Played for laughs. "Weakness to corruption"