Noctis, crown prince of Lucis, is Luna’s betrothed and the protagonist of Final Fantasy XV. As mentioned in the introduction, the premise of the game is that their wedding is part of a peace treaty, and Noctis is travelling to meet with her.
Though it’s an arranged marriage, it’s not one either party is opposed to: Noctis and Luna are childhood friends, though they have not seen each other in twelve years, since Luna’s home of Tenebrae was taken over by the Niflheim empire. The game doesn’t do a particularly good job of explaining how they came to know one another, leaning instead on the anime OVA Brotherhood to provide context: Noctis was gravely injured as a child and came to Tenebrae to recover. During this time, he and Luna became fast friends. (Side note: you should not have to watch a freaking anime for this kind of exposition. Thanks Square.)
In any case, there are a few flashbacks during the game to their time as children, one of which is an exposition dump of its own:
Luna: The Crystal was gifted to mankind, that we might know lasting prosperity.
Noctis: If the Crystal belongs to everyone, how come only Lucis gets to use it?
Luna: The kings of Lucis do not simply use the Crystal, they also protect it.
Noctis: Wait, so my dad’s guarding it?
Noctis: I had no idea.
Luna: To crown the King of Light is the calling of the Crystal. And keeping the Crystal safe until that day falls to the line of Lucis.
Noctis: And I’m the Chosen?
Luna: Yes. Only the True King, anointed by the Crystal, can purge our star of its scourge.
Noctis: You really think I can do that?
Luna: As Oracle, I will see to it. To aid the King is the Oracle’s calling.
Noctis: Then I guess I can do it. I won’t let you down.
Luna: I know you won’t.
Keep in mind that they’re children in this scene — Noctis is about eight, and Luna’s twelve. Precocious doesn’t even begin to cover it.
Still, this scene effectively covers the scope of their journey in the game. Even at this young age, they’re both well aware of their roles: Luna is the Oracle, and Noctis isn’t simply the crown prince, he’s the Chosen One, the King of Kings, the only one who can set this world to right.
(Why Noctis is so special is literally never acknowledged, because that’s how FFXV rolls. Seriously, he’s just the last of a long line of kings and queens. No reason is given for him being ~The Chosen One~ other than the game being about him.)
Snark aside, it’s very telling of his age that Noctis is so quick to accept what Luna is saying and promise he won’t let her down. He’s eight years old and has no grasp of the actual weight of these words. As a young man of twenty during the main game, Noctis spends a significant chunk of the story avoiding his responsibilities entirely. (Both figuratively and literally, depending on how much sidequesting you do.)
The same isn’t true of Luna. Even at twelve, she doesn’t sound like a child at all. I actually find the voice acting for child Luna a really interesting choice: while Noctis sounds like a kid in all of his scenes, Luna doesn’t at all. It’s really hard to get dialogue for children right in general; they often sound too young or too old. And while I don’t necessarily think this is great dialogue (because it isn’t), the intention to make Luna seem older comes through very clearly. Even at her age, she’s fully grasped her role as Oracle and what it entails. She’s composed and well-spoken, a trait that carries into adulthood.
Unfortunately, the happy days with Noctis were not to last, and when Niflheim attacks, Luna is left behind. She continues to communicate with Noctis through messages in a book carried to and fro by her magical dogs. This doesn’t make any more sense in context considering this is a world that has cell phones.
Still, when the game starts, it has been twelve long years since they last saw one another. The fall of Insomnia comes with the news that Luna died in the struggle, though in truth she made it out safely (see Kingsglaive).
Thus begins Luna’s journey alongside Noctis’s: the journey to awaken the Six. Her duty as the Oracle requires her to seek out the gods of this realm and bid them to make a covenant with Noctis, so that he might use their power. Luna’s journey is largely unseen in the game, but instead guides Noctis’s own path.
Yet, as Luna tells Gentiana, this is not what she seeks to be for Noctis:
Gentiana: At first, the father had mourned the fate of his chosen son. Yet in Tenebrae, the two found solace. It was not the Oracle who assuaged their fears. But the girl, she holds the true power.
Luna: I have little to offer a king, other than the voice afforded the Oracle. Nevertheless… And— I’m afraid you might find this foolish… But… to be together with Noctis again, if only for a short while… It would mean the world to me. I do not seek to guide him, merely to stand beside him.
Luna does, in fact, spend most of the game guiding Noctis’s path, but what she actually wants is to be together with him again. It’s a painfully human desire, one that doesn’t mix well with the otherworldly duties of the Oracle, yet she can’t help but want.
Along his path, we see Noctis question and hesitate, we see him try to avoid his fate. Luna does not do this: she holds fast to her duty. Only once, in a conversation with her brother Ravus, does she attempt to put her duty aside:
Luna: I beg of you, please, see the ring to Noctis on my behalf. Already my flesh has begun to fail me.
Ravus: No, I cannot accept it. By your hand it must be done. To deliver the ring and inspire the king is your calling. You mustn’t fall.
Luna: But… I lack the strength to go on.
Ravus: Find it, Lunafreya. You have the will. Go to Noctis. Show him the truth of your heart.
Again, I think this is Luna’s humanity shining through. She wants to reunite with Noctis and for them to be wed, but she’s spent her whole life putting her own desires second and her duty first. It’s not an easy choice, but Luna is guided by the concept of the greater good being more important than her own happiness. Everything she does is centered around that. Noctis, on the other hand, has to grow up and accept his own duty.
Of course, everything comes to a head in Altissia.
Luna sees Noctis again at last during her speech, but simply gives him a nod, which he returns. At this point, they both know that they have their roles to play here, and they’re both prepared — or so they think. Leviathan is an incredibly powerful goddess, unwilling to be tamed, and though Luna’s trident grants her great power, there is only so much she can do.
Especially when Ardyn comes and shoves a knife in her side.
Even mortally wounded, though, Luna calls upon the Kings of Lucis to come to Noctis’s aid so that he can bend Leviathan to his will. And when Noctis succeeds, Luna uses her magic to heal his unconscious form, rather than herself, and curls around him protectively.
The battle won, Noctis has a vision of Luna in a field of sylleblossoms, the two of them children once more. After a few moments she appears as an adult once more and bids him farewell, returning the Ring of the Lucii to him. Noctis tries to reach her, to no avail.
The symbolism is, uh, fairly heavy-handed, and I don’t really need to spell it out for you, but it essentially encapsulates their journeys. From the start Luna has been the one ready to accept what fate had in store for her and thus did all she could to ensure Noctis’s ascension. Noctis, on the other hand, is still a child even at this point of his story, and only reverts to his adult form at the end of the scene, as he tries to reach for her.
This is not a story where the prince saves the princess. This is a story where the princess saves the prince — for a time, and for a cost. Luna’s journey ends here, but Noctis’s continues on, even making a brief stop in Tenebrae to learn more of Luna’s history. And eventually Noctis comes to accept his own fate, though not without enduring struggles of his own.
At the end, after the Chosen King rids the world of darkness, the Lucian throne room is filled with flowers, decked out for the wedding that never happened — and on the throne sit the bride and groom.