In life only death and taxes are inevitable, and even then death is negotiable. Hikaru/Sai with past Hikaru/Akira, cameo appearances from at least two other series, and Sai-related spoilers.
A month ago he might have found himself in the midst of a Tanabata festival, but today everyone is going about their business and no one really notices when he digs out a scrap of paper and a pen and writes "I wish I could see Sai again" in small characters. He spears it on a low branch of one of the trees around a Buddhist temple and then leaves, holding onto the hope that his wish will be granted until he falls asleep a little before midnight. He dreams of a butterfly made of two paper fans.
It's the thirteenth of August now, and the rest of the country is celebrating Obon. Tokyo observed it the month before, though, so Hikaru just plugs in a string of electric paper lanterns hanging over his window before he goes to bed and falls asleep listening to Bad Luck's ridiculously addictive new stuff.
He dreams that the butterfly made of two paper fans is flying back to him, and when it lands on his hand there is a tiny person on its back with long hair and old-fashioned clothes. The person jumps off of the butterfly and grows up to full size, and it's Sai, looking shorter than he used to (although it's just like with Akari, Hikaru's the one who's grown taller).
Hikaru launches himself at Sai, knowing that in this dream world he can hug his ghostly friend as if they were both flesh and bone, but as he clings and cries and is generally entirely blissful he feels sleep falling away far too soon. Sai is only his mother, coming to wake him for breakfast and finding herself accosted.
Mrs. Shindou doesn't dress like that, though. It's really Sai.
"You made a wish, and your wish was heard," Sai says, and doesn't mind when Hikaru sobs like a baby missing his mother.
Hikaru's mother has gone grocery shopping, but neither Hikaru nor Sai is aware of this so they dress Sai in clothes Hikaru hasn't quite grown into yet and concoct a story where Sai is a friend from one of the go clubs and crashed on Hikaru's floor after a costume party the night before. The kitchen is empty, though, so Hikaru makes a quick brunch for both of them. (Sai's stomach growled on the way downstairs and it is part of the Obon tradition to offer food to the dead, even if they're technically alive at the time.)
When they've finished eating Hikaru and Sai go shopping, trying on clothes and making Hikaru feel stereotypically homosexual for the first time since he realized that girls weren't doing it for him. Sai ends up with several nice things that look good and actually fit him, though, so Hikaru figures it's worth it.
They have ramen for lunch, then go to Touya's go club to finally satisfy Akira's curiosity.
"I'd love to play you at last," Akira says, noticing the palpable joy surrounding the other two.
"Who can resist a game of go against the great Touya Akira?" Sai replies, and the two begin a game that soon has the attention of everyone present. Despite the fact that this is a game between two legends, no one remembers afterwards who won: all they remember is the fierce energy akin to any of the Shindou-Touya games where they ended up yelling themselves hoarse at each other over who was the more brilliant player.
As they are leaving afterwards, Touya pulls Hikaru aside. "He's not just playing along for you like I was, is he? You two are really involved?"
Hikaru is confused for a moment before realizing what Akira is asking. (Playing go may have sharpened some parts of his mind, but others remain as dull as ever.) "He's a ghost, Touya, not my boyfriend. We don't even know if he'll be able to stay through the night of the fifteenth. I'm just letting him properly experience the world of the living while he's got the chance." The beans are spilled just like that, but Touya is too confused himself to stop Hikaru from going when Sai comes back inside to hurry him along.
"Something that has never really made sense to me," Sai says as they play Spot the Fakes at an antique store, "is what sorts of things two men would do together. That is, you can't really lay with another man the way you do with a woman, can you?"
Hikaru has heard essentially the same question from almost all of his other friends already, so he only blushes at having to answer such a question in public in such an outwardly conservative place.
"There are all sorts of things I can do with other men. Not that I have yet, I told you before I've only really been with Akira, but I've looked things up. You know about the prostate, right? The gland guys have?" Sai nods a bit, recognizing the term from Hikaru's health book, and points out a vase that is most certainly not a thousand years old. "Well, appropriate stimulation of it gets most guys off, so with a little stretching and plenty of lube two men can have mutually pleasurable homosexual sex."
"Oh, Hikaru uses such big words now, I really have been gone a long time." Sai smiles innocently as he says this, and it's only because of the wicked gleam in his eye that Hikaru knows he doesn't have to resist the urge to hit him with his fan.
Pretty much everyone Hikaru introduces Sai to over that day and the next thinks they're boyfriends; even a few of the people who don't actually know Hikaru is gay see how happy the two are to be together again. Hikaru has a go match on the fifteenth against Kurata, who comments about how wonderful it is that adorable little Shindou-kun has found such a compatible person to be his significant other. Sai is too engrossed in the handheld electronic go game Waya lent him to correct Kurata, and Hikaru has heard this so many times in the last twenty-four hours that his protests aren't even half-hearted anymore--more like quarter-hearted. Maybe eighth-hearted, even.
Ochi just sneers a bit and goes off to do whatever he does when he's not playing go. He still hasn't had a proper growth spurt.
After the match, Hikaru and Sai spend the day talking in a park. They talk about distant places that Sai would like to visit if he had the time, foods he would like to try, games he'd like to play. As the sun gets low in the sky they begin to talk about other things--what Hikaru might do with the rest of his life, what the afterworld is like, ways they might be able to communicate. They go back to Hikaru's house and Sai changes back into his (very) old clothes while Hikaru does as he asks, making a small origami boat and taking a birthday candle and lighter from the kitchen.
They sneak out and go to the nearest acceptable body of water, a large pond they passed earlier, and the paper boat crumples in Hikaru's hand as he hugs Sai for what is probably the last time. They could never even do this before, and now that they can Hikaru wishes it had always been possible.
"I'm so glad we were able to spend this time together, Hikaru," Sai says, and his words make a warm spot on the boy's head. He continues speaking, about all the things he wanted him to do, but Hikaru isn't really paying attention. "And most important of all, I want you to find someone who makes you happy. I won't be around to constantly nag you, you know, so you'll need to find someone else to keep your best interests in mind. And maybe someone who's interested in go, shared interests are always good--"
"Sai, shut up," Hikaru says, and pulls away just enough to kiss him. He's fading already, though, and when he responds his lips are marginally more substantial than mist.
"I'll try my best to come back to you." Sai brushes his hand against Hikaru's face, watches as his fingers pass through. Hikaru is blinking back tears now, and looks down to make the paper boat water-worthy again. He sticks the candle in and lights it on the second try, then turns and puts the boat in the water. It draws Sai like a fisherman reeling in his catch, and though he vanishes from sight as the tiny boat sinks, Hikaru is pretty sure he can still feel the moisture from his lips.
Over the next eight and a half months, Hikaru continues. He plays go, goes to school, hangs out with his friends, and occasionally allows them to set him up on dates with other young men. He never forgets Sai, though, and none of the potential boyfriends last very long.
When the fourth of May arrives, though, Hikaru has a very odd feeling. He goes around in something of a haze, unable to focus on any one task too long and convincing plenty of people through his behavior that he's more stoned than a rock quarry. A group of four middle school girls stands near him on the bus, and the one with long hair and a seasoned look in her eye tells him he should be spending time with whoever's making his heart ache so badly instead of pining alone. Hikaru smiles and thanks her, but explains that it's hard to be with the person you love when that person's dead.
She smiles back sympathetically and says she knows what he means, and her name is Higurashi Kagome if he ever wants to just talk.
That night Hikaru goes to bed without dinner, telling his parents he's not feeling well. He dreams of a butterfly made of two paper fans for the first time since before his birthday, and he feels the kind of immense hope that can shatter a person's soul if betrayed.
When Hikaru wakes up, arms draped in familiar white are holding him. "They've allowed me to stay and live out my life," Sai says, and Hikaru feels like the happiest guy in the universe.