Angst? What Angst?: Adair has a noteable instance. It's possible, if unlikely, for Arisse to rebel after Elodie and Adair become betrothed. Assuming you win Arisse and several members of his family will be executed. It's likely Elodie will have to kill Kevan as well, possibly through a horrfifc display of fire-magic. How Adair reacts to this will never be discussed or touched upon, and he will still end up marrying and having a good relationship with Elodie regardless.
Awesome Music: Togami's theme, the ending theme, and Joslyn's theme.
Crosses the Line Twice: The game is full of it, the most obvious examples being Elodie's various deaths which are always accompanied with cute little Magical Girl style graphics depicting her demise.
Shipping Elodie with Charlotte is common, despite them being related.
In some circles, Elodie/Togami. Elodie being able to propose marriage to himnote an option only available if she doesn't have enough knowledge in Intrigue or History to know he's already married but for it to never be accepted can come off as something of a Yank the Dog's Chain for some shippers.
Charlotte/Adair is a downplayed example. It has been showing up more in fanworks, though (currently) always as a background detail and/or as a Ship Mate to another ship. They're probably paired together due to being two of the few unambiguously good people in the game, their proximity in age, and the fact that, as the queen's cousin, Charlotte would likely be the best marriage prospect for Adair after Elodie (as he's the last of his line).
Game Breaker: Obtaining the Lumen crystal early. Elodie as a Lumen can learn to control her magic, which makes several events that can easily end in her demise result in her victory simply by using magic to incinerate or drown someone.
Iron Woobie: Arisse. Landed with a Serial Killer stepson and then a vile husband who's responsible for how fucked up Kevan, Corisande, and Thaddeus are. Her non-murderous twelve-year-old stepson is being threatened by a foreign country. Her granddaughter is suffering under a dark secret that affects her parents. And now she has to deal with a queen who may or may not fuck up the entire country.
If Elodie was cruel and executed everyone that could be executed, there will be long stretches of silence in the game, and she'll reach coronation horrendously depressed and alone. She kinda deserves it.
Brin is trying to start a war for some reason, and when it goes wrong she expects Elodie to bail her out. However, the fact she has not one, not two, but THREE lovers dump her - the latest of which thought she'd be together with, dumped her for being "too challenging", and ran off with a priestess... ouch.
Kevan, Earl of Io. Hotheaded, abrasive, jumps to flimsy conclusions and is willing to murder for them. Being "seduced" by his stepfather and then having this be discovered strained the relationship with his mother and brother, to put it lightly. The subsequent refusal to speak of the matter and the assassination of the stepfather left him with severe trust and familial issues, which he eventually confesses to Elodie, should they marry.
His half-brother Thaddeus also counts. Elodie, who's seen how he behaved at school, is incredulous when his mother describes him as "fragile," but walking in on his father with his brother, followed by his father's death clearly messed the guy up. Furthermore, if Elodie marries him and talks Togami out of invading, he assumes his new wife is having an affair, and the game notes that he wouldn't have said anything about it had Elodie not figured out what was wrong. Considering what happened the last time he was witness to an "affair", can you blame him? Then there's the ending where Elodie and Banion really are sneaking around behind his back, culminating in a case of Mama's Baby, Papa's Maybe where he adamantly refuses to suspect her and makes people who question the baby's paternity in his presence regret it, for fear of how rumors will affect the child.
Jossed: While there's plenty of reason to suspect that Talarist had a hand in Adair's death, occasionally fans give him the blame for personally being behind the deaths of Adair's aunt and grandmother. In the final plot update, if Elodie has high enough intrigue to suspect him of the former, the narrative will outright say that the latter happened when was "too young to have had any possible involvement." Supporting this is information that has always been in his dossier and in the high-level Foreign Intelligence lessons: the suspicious deaths of Adair's grandmother and aunt happened twenty years before the start of the game. Talarist is currently twenty-six. He would have been six years old at the time of the deaths.
Even if Talarist couldn't have done it, that doesn't mean he didn't know about it. The string of conspicuous deaths in Adair's family and their line of succession, it's possible that Talarist's family has been trying to gain a proper foothold in Nova for generations.
Moral Event Horizon: The nature of the game means that you may or may not ever get to see enough of a character to find out how far they'll go (and even if you do, they'll probably be wearing a placid smile the entire time), and there's a whole lot of grey morality happening amongst the nobles of Nova, but honorable mentions include:
Kevan, the Earl of Io. His first appearance in the game is demanding a criminal be killed for trying to poison his sister; make a misstep and he'll pull a knife and cut her throat in broad daylight with no fucks given. He remains dangerously unstable even if you manage to have a good relationship with him.
Talarist, who may be willing to kill an innocent child to get what he wants. He denies that it was him, but since his bloodline makes him the rightful heir to Adair's lands and title if he dies, he's the only person with a motive, and if someone else is chosen to succeed Adair he throws a fit. Oh, sure, he comes off as nice and generous in person, but several of his actions indicate that he's a vindictive, nasty Jerk Ass underneath.
Banion is charming and confidential unless Elodie decides to punish his sister Brin for the Ixion mess. Depending on the exact outcome he variously resorts to sending an assassin, manipulating her into an engagement, calling a vote of no confidence against her, dueling her to the death, or pretending to marry her out of love and then poisoning her.
Togami, who goes around killing Lumens just to collect their crystals (which he can't use himself). It's possible to avert this trope with him, but redeeming him (the Save the Day with the Power of Music achievement) is a difficult Guide Dang It.
Elodie can cross a few over the course of the game:
Escaping the Old Forest alone and leaving Briony there to be horribly killed by tentacles—or, knowing full well how dangerous it is, telling her to go there alone.
Feeding poisoned chocolates to people (or, if your Animal Handling is low enough, cramming one down a chicken's throat).
If a servant eats the poisoned chocolates and dies, Elodie happily remarks that she's glad it wasn't her if her Cruelty stat is high enough, indicating she's already crossed it.
Sacrificing Charlotte to the kraken.
And, naturally, killing her own father to prove allegiance to Togami.
And finally, Lucille. If Elodie gets too close to figuring out all those assassination attempts, she'll immediately throw her own husband under the bus to deflect suspicion and then poison him so he won't talk.
That One Sidequest: Joining Briony on the adventure into the Old Forest. The number of skill checks needed to both open it up and survive it are so high and so numerous, that you pretty much need to plan for it from the very beginning.
If you misallocate Elodie's skill points, she can make some very stupid decisions, such as refusing to fund a printing press or a field hospital and trying to push out an arrow after being shot.
The chocolates deserve special mention. It's attached to several possible skill tests to unlock the option for her to do anything other than glut herself on poisoned candy. And you still get the option to eat it, even if you pass them all.
At the beginning of the game, her maid knows more geography than she does. As to the geographical topic in question, Talasse borders Nova (and Sedna borders Elath), so it shouldn't be exactly obscure knowledge.
When it comes time to adjust the royal budget, it now requires an accounting skill check to decide to just leave the tax rates alone. Otherwise Elodie blindly messes with things she clearly doesn't understand.
Elodie herself if you keep the Cruelty stat low. Everyone wants her head on a platter, their repeated attempts to kill her maim and hurt people close to her, and she's expected to be Queen despite not having the slightest clue. We would give her a hug, but she might interpret it as an attempt to strangle her.
Potentially Alice. As a servant to Elodie, she is at the mercy to the latter's whims, and is a frequent victim of Video Game Cruelty Potential. Elodie can beat her for reasons that aren't Alice's fault, dance with her at a ball which puts her on the spot with the scheming nobility, or even turn her into a Sex Slave.
Charlotte, depending. She can get bitten by a poisonous snake... set by her scheming mother to try and get her cousin killed, or Elodie can kill her mother - one method, by frying her alive in self-defense, or Elodie can have Charlotte executed along with her mother because of her mother's crimes, or, if you're feeling particularly sore about her mom, Elodie can have Charlotte fed to the Kraken, by having people tie Charlotte up and gag her first and then dumping her off a ship, after some horrible ritual has been performed on her. And all this is happening to Elodie's favorite cousin. Even worse is if Elodie killer her mother in self-defense - she'll be absolutely heartbroken and hateful towards Elodie, who is just as heartbroken as she is.
Briony has to deal with her parents never paying any attention to her, on top of the hideous secret that her mother and uncle are still reeling from their abusive stepfathers, and once were in a physical relationship. All she wants is someone to pay attention to her. Said methods may get her and her friend (Elodie) killed... if said "friend" doesn't leave her to her fate.
Joslyn. First he loses his wife, then his daughter is called on to rule even before she's crowned and is likely to die before the year is out, he's being pressured to provide an heir to his own estate while he's mourning, and may possibly be killed protecting his daughter from the man who murdered his wife, left in a near-vegetative state by the battle if he survives, or worst of all, murdered by his daughter-turned-evil.
Uncle Laurent, depending on your path. His wife is fully willing to slowly kill him with a mind-destroying poison to keep Elodie off her scent, and his own niece might be willing to kill his entire family for his wife's crimes. We don't see much of him in the game, but Laurent is basically Lucille's puppet, and there's a good chance he has no idea.
Corisande. In addition to her failing marriage and role in the Lillah-Mead family drama, she's likely to lose her daughter and her brother within a few weeks. If Elodie blames her for the former, she breaks down crying.