I headcanon that Riku smokes. Not because he thinks it’s cool or whatever but because it started out as a way to compensate for the pressure darkness was putting on his nervous system.
It starts when he’s placed with Captain Hook. Ends up taking a couple of puffs off of smee’s pipe when the pirate sees that Riku is shaking after summoning a bunch of heartless for practice. He’s not exactly caring for Riku, but he does pity him a bit.
Riku hopes that he’s kicked the habit while in the realm of darkness. Mostly because of the weird nicotine withdrawal dreams he keeps having. But when he gets back, and after he devotes himself to watching over Sora, one of the first things he does is go out and buy a pack of cigarettes.
He ends up becoming a chronic chain smoker for a while. Easily burning through 4 - 5 cigarettes in one go. He usually he’s either up on the roof of the mansion with Axel or he’s taking a few quick drags before he does something he really doesn’t want to do. If anyone (*cough* Sora) had been paying attention they could have tracked Riku through the amount of Cigarette buts he left like bread crumbs.
After the events of KH2 Sora catches Riku up on the tree house one night taking a drag. He’s been having nightmares since the realm of darkness and smoking after the stress feels like the only solution. Sora doesn’t get upset or judge Riku. Doesn’t even make him promise to quit, he just sits with him and lets Riku finish the Cigarette.
They keep it up for a while. Riku smoking through a pack while he shakes through the memories of everything he’d had to do, and all the things he is more than ashamed of doing. Until they get called to YenSids tower.
Before the Exam Riku steps out for a smoke. Mickey (though disapproving of the habit.) already knows he smokes, and lets him leave the pre exam meeting with little fuss. Sora comes out after a while and takes the cigarette out of Riku’s mouth and says:
“Promise me this is the last one? If we pass this exam, promise me you’ll quit?” And all riku can do is nod, and stub out the cigarette.
He does really well. After the mark of mastery and everything that goes down in KH3 he doesn’t touch a single one. Goes completely cold, and while it’s tough he distracts himself with training and missions.
… and then Sora disappears and Riku works really hard not to fall back on old habits, but he walks out of Merlin’s one night to find Cid, Lea and Isa smoking on the steps to the Bailey. Without speaking, he takes the packet out of Lea’s top pocket and lights one up with a small fire spell.
All he says is “I’ll buy you another pack,” and Lea can only shrug with a sad look in his eyes.
“Don’t worry about it kid,” he murmurs. “You know your braking your promise though, right?”
“He broke his first,” Riku rolls his eyes back and blows out a lung full of smoke. “I can stop again when I find him, but right now my usual crutch disappeared into the sunset so I’m using this.”
“Okay bud, okay,” Lea soothes.
And then when Riku finally finds Sora, and they hug for the first time, Sora mumbles “you smell like smoke,” into his jacket.
Riku gives a hysterical laugh and says “I’ll quit again I promise,”