I always find it so funny when people try to argue that "Japanese doesn't have plurals, so the use of 人 (hito) in 大切な人 (taisetsu na hito) can just as easily mean 'precious people'." Like, there are ways to pluralize in Japanese, often times it isn't viewed as necessary because of context, but 人々 (hitobito) means "people" and is a common word, same with 人たち・達 (hitotachi). There's even a difference between the two:
人々 (hitobito) is talking in a more general, large quantity sense of people. Like, there were tons of "people" at the train station.
Meanwhile, 人達・人たち (hitotachi) is a pluralized form of "person" and is generally used when referring to specific people. It would be used to state that there were a lot of "people waiting for the express train" at the station today.
So, if Riku was very specifically talking about wanting to protect the "precious people" in his life, then the dialogue could have conveyed that in quite a clear manner by having him say 大切な人たち (taisetsu na hitotachi), but...the dialogue doesn't say that and we don't see Riku protecting anyone except Sora in KH3.
People will constantly bring up that he is trying to save Aqua when he makes this realization in the Dark Realm with Mickey, but...not really. Like, he is trying to find her and bring her back to the Realm of Light and he feels a sense of duty and obligation to help her, because she helped him in 0.2/During the end of KH1, but...His finding her. His helping her. His "saving" her is all part of an overall mission. It's more of a duty. And, ultimately, he doesn't protect or save her. Sora does.
Who we do see Riku willingly, without any duty or sense of obligation or mission behind it, actually stand up and protect? Sora.
He straight up sacrifices himself protecting Sora here. He doesn't do that for anyone else and KH3 ends with him being the one to go out and find and save Sora.
Then we have absolutely gorgeous and wonderfully made GMVs like this one:
That make Riku's character arc and build up to the KH3 scene very clear and easy to see and follow. Riku has always been about protecting Sora. In KH1, that desire to protect Sora was corrupted and misguided, more a desire to protect the bond that he had with Sora, rather than Sora himself. He thought their bond was "threatened" by other friends entering Sora's life and becoming important to Sora, but then from the end of KH1 onwards we see Riku really focusing on protecting and saving Sora.
So it makes sense that Riku would use 人 (hito) instead of 人達 (hitotachi) if he was talking about others in their group of fighters or even 人々 (hitobito) if he was talking in a more generalized "people" sense. He wasn't though and the dialogue makes that clear, as does the storytelling from KH1 - KH3 and, most importantly, the scene where Riku sacrifices himself protecting Sora.
Obviously, Riku has other important people in his life that he feels connected to and that he cares about on various different levels. But Sora is the only person that is both precious/important to him and one that he is shown actively saving and protecting.
