Lmao. What relationship? The title had me choke.
But considering the user seems young and genuine, I'm not going the sarcastic route here. Roxas and Naminè do have romantic subtext underlying their otherwise normal interactions (if you took said subtext away, it is actually normal). They even made sure to somehow implement it in Days by pulling the Girl of my Dreams trope again despite Xion existing. Then the 2.8. credits. There's a point the creator is making here. I just hope they handle it well in the future once they both come back.
There's a difference between loving someone and being in love with someone.
But as it is, it's still a newly created bond of friendship and Roxas and Naminè are not in love with each other (or harbour feelings for one another either). Attraction, interest, a spark, etc. Maybe. That's a more fitting description.
The "we can be together again!" is also a grossly misinterpreted scene. And rather than it boosting Roxas/Naminè, it was made to boost SoKai in my opinion. Naminè originally says in Japanese that "we can meet each other again!". While their time was limited, Roxas was in a moment of loss and needed answers. Naminè made a point of guiding him and halting Diz's cruel plan of making the boy return to Sora blind. Her role was basically akin to that of an angel (with the cinematic symbolism to match, as she had this ethereal quality in KHII). They had moments to talk, about deep topics no less, and got to know each other a little. They bonded through their similar circumstances. I feel that's what spurred Naminè's determination to help him. What was happening was unfair.
In the end, they got along well and liked the company each of them provided.
Naminè tried her best to reassure Roxas when Diz was manhandling her, and she wanted to say a lot more. At its core, she had to state that no, he won't disappear, and that no, he won't be alone, because even if they may not recognize each other, they will meet again (a.k.a. they'll be whole).
The scene in TWTNW continues this. Naminè speaks through Sora to Roxas, saying she thought Nobodies were doomed to fade. Instead, they're able to see each other as they remember one another to be (that was the basis of Roxas' convoluted jargon). They need one another to continue existing. Any time Sora and Kairi are together, they're together. And they aren't suffering through their fate alone. It's the only positive thing they have to hold unto at that point in the game, so they accept things as they are. Hence, "we can be together!".
So it's now co-dependency.
It's like a silver-lining.
KHII I believe was trying to conclude their stories and make them content with their end result. Which is why they look happy. Things have changed now though.
I know that people are going to compare the amount of time spent Xion and Naminè each had with Roxas. Xion wins in this case. Yet that doesn't mean anything if Roxas and Naminè are to return in KH3 and perhaps get to strengthen their friendship more. Or even beyond this if there's another saga. I doubt they're gonna be canon by the third installment. Seriously doubt it. Too much stuff to cover.
Nonetheless, you can know a girl/guy friend for years, but they meet someone else one night at a bar or something, and have a romantic connection with that person that they didn't have with said friend. It's real life at work here. So Naminè doesn't need to have known Roxas for almost a year to make sense of them being romantically linked together.
Whatever happens, good luck with the ship wars, if any.