Can’t do that. They’re the heartbeat of the story because they separately and also jointly move the plot along so you’re better off asking questions.
But I can say this. Even if they’re not romantic (which I don’t think they are yet) their relationship is the most moving in the novels. Those who cry ‘omgg can’t believe xx is the ML I’m dropping this’ *shrugs*. TBH sucks to be them, they won’t be experiencing one of the greatest reads in modern isekai.
Looong spoilers
I should mention that even by the end of the novel, things have ‘not yet blossomed’ romantically between them. Myne’s love for books is the very core of her character, and though she admits that Ferdinand is more important than her library (a very big thing!) her feelings have yet to catch up to that point (romance). The thing about the world in AOB is that engagement/marriage is hugely a means to gain political favor, and we cannot apply our modern values to it. It is an alliance between your noble families to help your standing. So if Ferdinand / Myne becomes engaged, this is less to do with any romance but more for the sake of the nations (it’s a very long story).
Ferdinand himself has feelings, developed only at the final volume, but even that (for me) doesn’t compare to his desire to experience what ‘family’ is like, knowing what he knows and feels when he sees Myne with her commoner family. He was greatly abused as a child, and this shapes his character and beliefs early on, being guarded, perfectionist and stern. Rosemyne has completely changed this, in that he realizes there IS someone who truly cares for him. So if you were to ask me, are they in love with each other the way we expect? The answer is … no. Not yet. Even if they may be engaged to each other, they are not yet at that level. Ferdinand is fast catching up tho.
Anyway, to answer your question, there are two turning points, ALL near the end of the novel. It is a very long way if you compare this manga.
The first turning point for their relationship was during the events near the end of Volume 4, where Ferdinand was ordered by royal decree to wed Detlinde, Myne’s schoolmate (and one of the antagonists), in another nation, being the perfect candidate to stabilize the political climate there. This meant Myne and Ferdinand will be permanently separated… no longer will he be there for her, to act as her guardian, to watch over her health. Myne was coming to terms with all this. In the end, they both exchanged gifts that shocked their closest aides present with how intimate it was. Ferdinand’s especially, was a carefully crafted hairpin of rainbow feystones of the highest quality, meant to protect Myne in his absence. (For Detlinde earlier, so disinterested was he in giving a proposal feystone that he gave her one of his random creations and spun a sweet tale of bullshit which she fell for.) Here, this is where she reveals to Ferdinand that he is truly family to her. The blessing she gave to him was of the highest of high, never seen before. It’s a testament to how much Ferdinand means to her. I cried at this entire farewell scene.
The loss of Ferdinand here means Myne has to learn how to navigate her way politically and juggle all her duties and even more in Ehrenfest. They do exchange letters at Myne’s request, more to reveal enemy movements in Ahrensbach, this is why Ferdinand chooses to remain there and go through the wedding. It’s so he can protect Ehrenfest from there. They will have one more chance to meet again later for one day, during her academy events. Thing is, everyone close to them knows this is how they are… Ferdinand worries over Myne, checking over her health, touching her wrists and feeling her forehead is so second nature to him. Myne in return also seeks out Ferdinand, interacting with him very closely, they bounce off each other also with snark and how she’s pretty much the only person who could tell him off at that level, and him to her. Things like this are typical for them, but to the public who saw this then, this is an alarming closeness that is like an engaged couple (in a bad way because she is already engaged to Wilfred by that time. It’s like why the effing hell are you more worried over Ferdinand than your own fiancé??).
The second turning point was when Detlinde was going to kill Ferdinand that he gasped out Myne’s name and in Ehrenfest, she instantly saw a vision of Ferdinand losing his life. (Remember in the early days when Myne called out Lutz’s name and he could see what’s happening? It’s the same phenomenon.) Myne went BATSHIT insane, like she literally INVADED another country just to save him. I quote her :
“… The greater duchies, the Sovereignty, the royal family, and even the gods themselves—I would make an enemy of the entire world to save you.”
By this time, only Ferdinand has not seen her in her full adult body yet since he was in Ahrensbach, so he couldn’t recognize her. Anyway long story short, I believe this is where he started to see her as a woman (and the launch of a thousand fanart) I’ve tried my best to keep things as vague and less spoilery on plot so I hope this helps.
In AOB world, there are 420 days in a year (maybe best to refer it as seasons instead of year, since a new year starts in spring and ends in winter). She is 15 years old by the end (officially 14 on paper because she lost a year when she became Myne) Ferdinand is 13 yrs older.
So in our standards aka earth’s time, she
will be older than that, hovering 18 yrs give or take.
Ask me anything on romance if you want. Sit down and be ready to experience one of the greatest isekai world building works and an equally mind blowing relationship development between two certain characters. They are on top 5 of most popular pixiv pairings. I’m ready to defend the pairing. If you stop here because of that, make no mistake - you will lose out big time. Tbh I can’t even call it romantic. But they are shades deeper than many pairings out there.