The place reeks of darkness, thick and suffocating, a biting, metallic scent that makes his nose curl and his face pull into a grimace of disapproval and hatred and everything else he feels when he finds his senses overwhelmed by that scent, by the sensation of the darkness. It's an artificial stench here, not like the real darkness that he's experienced, not like how he can taste the real darkness on his tongue and smell the real darkness on his own skin, but it's still darkness, nevertheless, and it makes him want to gag, but he holds it back, and follows Sora deeper into this place, and doesn't say anything about the stench, and pretends he's perfectly fine.
They're here because Sora said he wanted to visit someone before their big, final battle. Sora doesn't like to call it "final," because it makes it sound like they're not going to come back from it; Riku doesn't really think they're going to come back, even if Sora doesn't say the "final" part on the title, but he doesn't say anything, because he likes to let Sora think that, maybe, they will make it through, and he likes to think that, if Sora thinks it, they just might. He's not keeping his fingers crossed, though. He knows they don't really stand a chance against this, no matter how strong they've both become.
Sora leaves him to sit in the study of some room in some tunnel, and tells him he'll be right back, he just wants to talk in private for a minute. Riku can't help but feel a little pang of jealousy when Sora walks through the door in the back and vanishes off into the metal-lined corridor to somewhere he doesn't know and can't see, but he doesn't dwell on it, because he knows it's really not worth it. Jealousy it what got him into his mess in the first place. He doesn't want to repeat that again.
The study has such a heavy sense of the darkness that it's making his tongue feel thick and sticky with the stench and taste of it; it's so deep in the back of his throat that he has to breath deeply, in his nose and out his mouth, to prevent himself from gagging on it. He thinks that this is probably where the darkness first came from, knows this is where it first came from, because the voice in the back of his mind that never really went away resonates with this place, screams out to him and tells him that this, this is where it all began.
He glances to the painting sitting on the ground, lopsided, like it had been torn down in a revelation of some kind. He doesn't recognize the face right away, but he knows who it is, and he stares at it until Sora comes back, his friend tailing behind him, lost deep in his own thoughts.
"Riku, this is Leon; he's the friend I was telling you about." Sora introduces them in that typical Sora style of his, all smiles and handshakes and "everyone can get along now, right," because that's just the way Sora is, and, even if Riku doesn't like this "Leon" person all that much, when they shake hands, they both pretend to smile and mutter a gentle "hello" under their breaths, just for Sora's sake.
They sit down together, and Sora and his friend talk for some time about nothing in particular, about what to do and how to protect himself and when to back out of a fight, and everything else that someone like Leon, battle-torn and aged with knowledge and fights he shouldn't have had to fight, should tell someone like Sora, new and fresh and a little naïve, but knowing almost as much as Leon does, if not more. Riku doesn't really pay that much attention, because neither of them address him, and when they do, he simply nods, lost in thought.
Riku wonders how someone can live in a place like this, knowing what happened here and having to smell that darkness all of the time, but he doesn't really want to ask, because he thinks it'd be the same if Destiny Islands turned out like that, smelling of darkness and making him almost gag every time he walked out to that island with the papou fruit on it, where it all started, where he almost ruined it all. He thinks he'd be in Leon's position if he were to go back and everything was destroyed: he'd try to rebuild it, put it back together, no matter how hopeless it was or how many Heartless there were, no matter what it smelled like or felt like. He thinks Leon must really not be that bad of a person, if Sora can sit there and talk to him and smile that pretty smile of his, even when they're about to go fight an uncertain battle, even if he doesn't really think they're going to come back, and he decides he doesn't really mind him when they shake hands one more time and they share the knowing look of two men who have grown up far too fast (he realizes how alike they are then), and Sora hugs Leon a little longer than he should, and Leon smiles a little bit into Sora's hair, something Riku knows isn't really like him.
When they separate with Leon, Sora seems reluctant to go, and Riku grabs his hand and pulls him along and tells him that they're off to fight their final battle. Sora gives him a little look that says "but it's not the final battle," and Riku simply tells him that it is the final battle, because, after this, they're gonna go home, and then they can rebuild everything there, just like Leon is rebuilding here, and they won't have to fight any more unless it's playful tumbles on the beach, just like they used to.
Sora smiles at him, knowingly, and he understands, and they climb into the gummi ship, and neither of them look back when they leave.