Well it's not Roxas because Sora lists Roxas before he talks about this other person. Some people theorize Xion but I don't really see that since they had like... all of one conversation and I don't remember Nami being all that close to her during it. Though them interacting in the ending could be a callback to this perhaps.
Well, to be honest, Sora also listed The King as one of the people that missed her when I can't recall them
ever meeting (lmao). Donald and Goofy don't remember Castle Oblivion. So it being Xion wouldn't be so out of place in this case with the other WTFs he listed. The "someone special" is more of a localization add on when the original is more like "someone also wants to meet/see you the most". I doubt Sora doesn't know Riku is aware of Naminè and vice versa, he could've definitely used his name.
It's odd that Sora feels Roxas and Ventus but there's no inkling of Xion ever despite that her heart is inside him. It'll also add more context to the Xion and Naminè scene. While I don't care for Xion, that was admittedly cute, and if there's anything we need in this series, it's female friendships. I'm all up for this theory for this reason. Naminè was the first potential girl friend she's met and while the guys are good and all, I wouldn't blame her to want some boobs to relate to. Notice how Xion in the last cutscenes interacted more with her and Olette than Roxas or Axel.
Also, as mentioned, it's hard for me to process that Riku would fit that line when there was no sign of him thinking or missing her like the other two (Roxas and Kairi), and he was focused on other priorities. Naminè didn't get to his mind until his replica emphasized that this vessel was for her. It's either something's off or it's bad writing.
That whole line with Sora was weird really (the Disney trio?
Really?). I'm hoping Nomura would provide some clarity on it. Not holding my breath, but hopefully.
[rubs temples] This game...
Spoiler
