No problem! This is a cute prompt.
His S/O would be the child of a government official that works with Regis.
They’d have been of delicate health and always end up coming with their parent to work with them, so they’d be near by if they fell ill.
It wasn’t unusual. Most Government official’s children came to work with their parents. It was safer that way. Normally they’d have designated areas to play and interact while their parents were in meetings.
None really took an interest in the prince though. And like all kids, they bully the ones that are different.
Noctis would have happened to be sneaking around and caught sight of them being bullied and stepped in.
He scares them off, and even managed to scare his future S/O in the process. But then he’d take them with him while he’s running around and kinda look after them.
They wouldn’t say much at first, which Noctis wouldn’t mind. It was sort of fun just having someone tag along. But eventually they’d open up during their adventures around the palace, and a sweet personality would be revealed.
They’d talk back and forth, and Noctis would become a great support for them.
(Meanwhile, running around with Noctis took a lot of energy, but they didn’t mind. And after a while, it even began to help.)
They’d eventually become close friends, but would be separated when they start going to school.
They’d be sent to different ones, and for a few years, they’d lose contact with each other.
It would only be by chance that in high school, they’d meet again. They’d still go to different schools, but they’d bump into each other during a class trips to a museum of some sort.
And they’d decide to relive their childhood by sneaking off together just to hang out for a while and catch up.
(The school would be forced to notify Ignis that, whoops, the prince is missing. Cue the largest headache he’d have in his life.)
They’d be sneaking around on top of the museum, because no one would really think to look on the roof of a museum.
His future S/O would be something of a dare devil, balancing on the edge of the roof with a sort of ‘Devil may care’ attitude.
He’d probably point out how much they’d change and they’d agree.
“I’m not so soft anymore.”
They’d open up about all that happened while they were apart, and share the things that bothered them.
(Though his future S/O would hold back, recognizing that he seems super stressed out by thoughts of the future, like they once had been, and just let him vent.)
They’d eventually climb down to the bottom of the museum. (Or warp down depending on if they could convince Noctis to try and warp with them.)
They’d leave, and pass right behind Ignis who was searching for Noctis inside the museum.
Noctis would grab their hand and they’d take off running before he could see them. They’d pass by Noctis’ car, as Ignis had driven it here, and see the car keys inside.
His friend would try and convince him to take the car, but he couldn’t do that to Ignis. He’d make him worry but stranding him at a museum was a biiiiit much.
….but he does park it in a different place.
So they take off on foot and head to a cafe and just run about the town going wild.
Eventually when the day is over, he’d finally take out his cell phone and see exactly 23 missed calls from Ignis, and 4 voicemails.
They’d do the same and see that their caretaker had called them three times to check on them.
Both would snicker and finally return their calls.
They day would end with them waiting in a park to be picked up and they’d wait together. Before leaving each other’s company, they’d exchange cell numbers to stay in contact.
His friend would let him call them at all hours of the day and just listen. And if it ever seemed like he needed a break, they’d tell him to meet up with them. (Which leads to some hilarious conversations when it’s like, 3 in the morning and they’re urging Noctis to run off with them to cause trouble.)
Prompto of course would be invited on these trouble making trips, but very rarely would he actually go. (He can tell something’s between the two, and doesn’t mind sharing Noctis’ time.)
Together they’d constantly worry their families and friends by disappearing, but it wouldn’t stop them. It was selfish of them, but they couldn’t help it. They wanted to spend time together.
Eventually, of course, that would lead them to dating.
Their relationship would be a very large stress reliever for Noctis. His S/O, compared to when they were children, was so strong now. And they’d have nothing but compassion for Noctis’ position as the prince.
Imagine Final Fantasy 