Riku was alone. Again.
He tried not to think about it too much; He had enough on his plate as is without angsting over things that couldn't be changed. Searching for Sora was top priority, and he couldn't afford any distractions. So instead of wallowing in his loneliness, he occupied his time with the search.
When he wasn't searching (there was so little progress, he felt he might go mad staring at the same results) he was checking in on the others. How were they coping, he'd ask, have they been having any issues as of late? It was hard, he knew, trying to live peacefully after the war. After so long fighting to survive and jumping between worlds, the idea of settling down for a calm life was. . . daunting, to say the least. The transition was especially hard for the ones with little experience existing period (he tries not to think of how much of that was his fault).
They all try to get along for Sora's sake. He wouldn't be happy if he came back just to see all his friends fighting each other. Everyone tries to get past their histories with each other, and turn over a new leaf. Riku tries, too, but light it's hard. It's especially hard when he sees Xion flinch away from her reflection (expecting someone else's face,) and Namine looks at him that way (looking for someone else entirely). It's hard, at times, when he's making up for lost time with Terra and he almost thinks he sees someone else (and maybe Leon's right, maybe he does need more rest if he's mistaking brown for white).
It's so hard watching Kairi lie so still, dead to the world. It's hard watching her chest rise and fall so slowly in her slumber. It's hard knowing that, even if he gave into impulse and tried to shake her awake, she won't so much as groan. (It's even harder knowing how little time he spends by her side, but the guilt is preferable to the phantom image of another time she slept under his watch.)
Being with Roxas, somehow, is less hard. The time with the boy is less painful than others, and easily kept him focused on the present opposed to the darkness of the past. Maybe it was the way he seemed just as hurt by Sora's absence as Riku was. Maybe it was the way he understood why Riku couldn't rest, why he couldn't take a break from the search for long. Maybe it was the way he understood how Riku could hope without hope for Sora's return. Maybe it was because, unlike the others, Roxas absolutely won't forget what happened between them, makes it known that forgiveness will only be easy, but accepts Riku's efforts for it regardless.
Maybe Riku's getting tired of everyone pretending everything's okay, and his honesty is refreshing just as much as it's painful.
Riku is equal parts respectful of and thankful for the man's grudge. Everyone else had reassured him that he was forgiven, that the past was the past, but Riku can't help but feel as if he'd yet to truly make up for what he'd done. Not to everyone. So, it's that feeling of progress, that feeling that he's putting effort into something that will come to fruition that draws him (especially since the search's progress hasn't changed since they first started.) Riku finds himself seeking Roxas out first whenever he feels that he can't take the situation anymore.
When he finds himself second-guessing the honesty of his friends' words, he goes to Roxas.
When he finds himself a hair's-breadth away from using the Power of Waking his damn self to see if it'll send him to the same place as Sora, he goes to Roxas.
When his frustration with himself peaks and he finds himself being bitter to the people around him, cursing how the Wayfinders are joined at the hip, how Isa was accepted into Twilight Town with open arms, how Namine was so damn peaceful in Radiant Garden, and cursing Kairi for leaving him alone to deal with all of this --!
He goes to Roxas.
Sometimes, they fight. They'll go to some unfortunate world that'd yet to be rid of heartless and let loose. They were past fighting each other, but they'd spar when not even the heartless was enough. They'd go back-and-forth, back-and-forth, at full strength, and the adrenaline would make Riku forget his troubles, if only for a bit. He'd admit that he missed fighting for the sake of fighting. He missed the days where he and Sora would play fight on the beach, but then Roxas would hit him extra hard, and it was pushed out of his mind.
Most of the time, though, they just sit in each other's presence. They simply co-exist for a moment. It was quiet, and relaxing, and always just the two of them. Riku would feel bad about taking time away from his friends, but he knows Roxas would tell him if it was a bother. They never took to the Clocktower, anyways, so it's not like he's really taking anything from them. They never visit Destiny Islands, either, so Riku can assuredly tell himself that Roxas isn't just a replacement.
(He wonders, sometimes, if Roxas was like him. Uncomfortable with the forced pleasantness oozing from everyone, and waiting for the other shoe to drop. Waiting for everyone to admit everything's not okay, and trying to be ready so it doesn't break him.)
One day, something else happened. He's not sure how it happened. They were on the Gummi Ship, heading back to Twilight Town after visiting Namine. The ship was on auto-pilot, what with the lack of heartless ships on this specific route. Roxas was standing as close to the window as he could, hands pressed to the glass. Riku understood the feeling, and remembered the awe he felt as seeing the Ocean Between after so long of using Dark Corridors to travel worlds. Roxas's face betrayed his amazement, and Riku was so drawn by it that he suddenly found himself right next to the boy. Roxas looked at him, then, with the stars reflected in his eyes, lips slightly parted. Riku couldn't tell you who moved first, but between one moment and the next their lips were together, soft and sweet and over quickly.
They said not a word about it afterwards. Not when they landed, not when they said goodbye, and not when they saw each other again the week after. They don't talk about it when it happens then, too, and the next time, and the next. They don't talk about how they slowly progress from dry and chaste to deeper and more passionate each time. There's nothing to be said. They understand each other well enough at this point that talking about it would be meaningless, and would never do it justice.
Riku understands, at least.
He's not quite as alone as he thought.