Can I engineer which couples end up forming after the final mission? If so how?

In previous Fire Emblem games (like Awakening), marriages between the characters happened during game, and they even unlocked paralogues. After completing the final mission and seeing the battle awards, there would be a short epilogue written for each couple and character. Married characters would be paired together. Importantly, it was the player's choice to arrange the marriage, and this happened within the normal support menu. I think there were some "canonical" marriages, but marriage dialogues were available for all valid couples.

In Three Houses, marriage happens exclusively after the final mission. The player-character can choose a partner before going into the final mission. But from what I can tell, the game automatically creates pairs of NPCs who have strong bonds. Like previous games, a short epilogue is written for each couple, but in Three Houses the marriages get determined by the game. I managed to get all my units to have maximum (A or B) relationships, so I can't tell if there are some "canon" that are preferred or if there are other ways the game prioritizes who ends up together.

Do the NPC relationships in Fire Emblem: Three Houses follow some sort of priority system that I could leverage in order to ship certain characters together? For example, can I exclusively only develop certain A-level supports to force the game into pairing the couples I want? Or are there canonical couples that the game prioritizes?

In case specifics matter, this is my final roster and units who ended up together in the postgame epilogues (minor spoilers):

I chose Golden Deer for this playthrough.

• Byleth (male), Lysithea (player choice)
• Caspar, Catherine (fellow "thunder" mercs)
• Bernadetta, alone (went back to being recluse)
• Claude, Marianne (sounded a bit shoehorned: they fell in love)
• Lorenz, Dorothea (celebrity marriage of the ages)
• Leonie, Seteth (bonded over fishing)
• Ignatz, Flayn (Ignatz promised to paint Flayn)
• Raphael, Hilda (open craft/accessory store)

So because I had an odd number of units, I assume that the game made Bernadetta the only single because she is the social recluse? Or recruited from another house?

Solution 1:

As you've likely already guessed, under the hood of the support ranks displayed in game, there's a numerical score for every support bond. What's not especially clear, is that, once you've reached the highest rank between two characters, this score can keep going up. Pairings at end game appear to be determined by this support value, rather than any particular narrative priority, or by the order in which various pairings reach max rank.

So, yes, you ought to be able to manipulate these results by grinding out support bonds between two characters in combat, or through meal sharing, etc. on exploration days at the monastery.

Solution 2:

Update since the DLC, minor (non-story) spoilers:

If you've got the Cindered Ashes DLC, you will have access to a new region in the monastery called Abyss, which you can upgrade. One of the upgrades which becomes available about halfway through the game is a fortune teller like character.

She gives you the ability to either increase your support between two characters, or make/break a relationship.

Making or breaking a relationship is a costly affair (10k renown), but it does guarantee the outcome you want.