This story is based on a VOCALOID song of the same name. If you have the time, I highly recommended checking either versions out.
VY2 (Yuma) version: watch?v=GdSUMFi5FYE
Miku Hatsune version: watch?v=rPAZO7PI7Y0
The First Year
Shintaro is fifteen years old when he first starts writing letters. He'd write each and everyday without fail, hunched over his desk as his hand swiftly wrote away his feelings. Sometimes, Momo would peek over his shoulder out of curiosity, expecting to find a bevy of new material to tease her brother with. But every time she attempted this, she would find herself choking back her tears, turning away in embarrassment before her brother could even catch a glimpse of her.
The Second Year
Shintaro is sixteen years old and writing with so much fervor that he fails to notice the rising smoke from his overheated computer. As flames rise and his mother and sister's cries of panic grow louder, Shintaro continues to dutifully write. Even when his clothes burn away and his computer, his lifeline, is engulfed in flames, he continues to write and write. It takes a hard slap from an exasperated fire fighter to snap Shintaro out of his musings as he hastily grabs his writings and is forcefully pushed from his room.
The Third Year
Shintaro is seventeen when his writing has finally reached a professional level. At Takane's suggestion, he posts a few of his letters on his blog online, and before long, he finds himself being scouted by several magazine publications. Momo's manager, not one to lose sight of any potential talent, quickly seizes the chance to try and get him to write songs for his sister. But Shintaro refuses. These letters are addressed to someone special, and only he can properly convey those words. Momo nods in understanding, as she drags her distraught manager away, allowing her brother to finish writing yet another letter.
The Fourth Year
Shintaro is eighteen when he starts writing for a love advice column in a popular magazine. He was hesitant at first, too embarrassed to have his thoughts publicized for millions to read. "But your writing's so good, Shintaro! It'd be such a waste!" Haruka cried, clutching the magazine close to his chest. Takane agreed, and not in her usual sarcastic way, for once. The pair eventually wore Shintaro down, even enlisting the aid of his sister. So Shintaro relented, allowing his writings to spread throughout the nation, hoping that his feelings for that special someone would spread along with it.
The Fifth Year
Shintaro is nineteen and captivating women of all ages with his writings. Book signings and autograph sessions have become the norm and Shintaro marvels at how Momo has managed to live such a life for so long. He's grown tired of the squeals and proposals, rejecting woman after woman, who all ask the same thing; "Who are you writing for?" Takane has taken to calling him the Heart Breaking Ikeneet* ("In case you haven't noticed, I have a job and I'm not in my room right now!" Shintaro angrily responds), giggling every time a fangirl furiously stomps up to them and demands to know if Takane is the girl he's been writing letters too. The giggles then turn into guffaws as Takane fumbles for a picture of Haruka on her phone; "No way! This kid's just my junior! Besides, I'm already taken!"
The Sixth Year
Shintaro is twenty and reaching his limits. He'd already written over 2,000 letters and not a single bone or organ in his body remained unbroken. His mother scolded him, his sister confiscated his pens and paper, and even Takane and Haruka discouraged him in his endeavor to write more letters. But despite lying paralyzed on his hospital bed, Shintaro continued to earnestly write to her, suppressing cries of pain as he continued to write using his pillow case as paper and a pen he had kept hidden in his shirt (there was no way Momo would ever resort to strip searching her brother). His joints continued to ache, and his bones screamed in pain, but Shintaro was content. As long as his words could reach her, Shintaro was content.
The Seventh Year
Shintaro is twenty-one and fully recovered. Naturally, the first thing he does once he's discharged is write. His thoughts haven't changed once as his pen continues to convey his feelings for her. He compares her to everything he loves, from the satisfying gulp of a newly opened can of soda to the soft feel of his pet rabbit's fur.
The Eighth Year
Shintaro is twenty-two and still writing letters. His sister has managed to find love on her own with a boy four years younger than her. Not one to miss an opportunity to tease, Shintaro finds himself making cracks over his sister's tastes in guys. "Sh-shotacon?! I-I'm! He's not a shota anymore! And he's almost legal! He'll grow soon, too! In fact, I'll bet he'll grow taller than you, onii-chan!" Shintaro chuckles as he pats his sister's pouting head and apologizes. Momo's changed a lot, having matured and growing taller, almost the same height as him. She quit the idol business after she and Hibiya became an official couple, leaving her manager and many dejected fanboys in tears. Shintaro's grown a little taller himself, but his feelings for her haven't changed one bit. As he hunches over his desk once again, he wonders what he should compare her to this time.
The Ninth Year
Shintaro is twenty-three and once again bound to a hospital bed. The doctors tell him that he fell from a flight of stairs after a long night of writing and severe lack of sleep. The fall had caused him to lose all of his memories, including his own name. There was but one thing he remembered, and that was to continue writing to the person he loved.
The Tenth Year
Shintaro is twenty-four and his memories still haven't returned. Haruka and Takane are getting married, and Haruka himself requested that Shintaro be his best man. Shintaro feels guilty, guilty for taking such an important position despite not remembering the years of friendship he shared with the soon to be wed couple. But the pair insists, and Shintaro relents, feeling almost nostalgic. After the wedding, Shintaro is greeted by an older, bespectacled man. "Shintaro, right? You still haven't got your memories back?" Shintaro nods awkwardly, eyes darting to the ground. The older man quickly apologizes, "Ah, sorry, sorry. Looks like I touched a sensitive subject. Anyway, I just wanted to thank you. For remembering her, after all this time." Shintaro looks up at the man quizzically.
"Ah, how rude of me. I haven't introduced myself yet, have I? I'm that blissfully happy couple's high school teacher, Kenjiro Tateyama."
"T-Tateyama?! Then you're-"
"That's right. I'm sorry she hasn't replied in a while. She's been really busy, you know? She's the type of girl that helps anyone she feels needs help. Heh. You know, when she was a kid, her dream was to grow-up to be a hero. Guess it came true in the end."
Shintaro can only nod, but before he can respond, he hears Takane yell out, "Sensei! You better not be teaching Shintaro anything weird!"
Kenjiro laughs, raises a hand in farewell to Shintaro, and heads to the newly-wed couple. "A hero, huh?" Shintaro chuckles quietly to himself before he wanders off to his sister, who's trying her best to convince Hibiya to dance with her.
The Eleventh Year
Shintaro is twenty-five and still hasn't regained his memories. He's still dutifully writing letters everyday, but today he's in the hallway outside of the emergency room, rubbing soothing circles on Haruka's back. "She'll be fine. Takane's strong." Haruka nods silently, but his shivers never stop. Finally, a nurse exits the room, her smile bright as she turns to Haruka. "It's a girl." Haruka smiles the biggest, widest smile Shintaro's ever seen him wear as the older boy rushes inside, eager to see his new daughter. Shintaro follows close at his heels, calling a brief thanks to the nurse on his way. Takane holds her daughter close, smiling as she gently cradles the infant in her arms. Haruka looks at his daughter with awe and wonder and slowly, hesitantly, comes closer before his wife tugs him over. "She's beautiful"; he whispers, "What should we name her?" Takane puts a finger to her chin, humming in deep thought. "Hmmm, how about Aka*?" Shintaro's not sure how, but Haruka's smile grows even bigger as he nods his head excitedly. "Yes! That's a great name! Ah, Shintaro! You should come see your new niece!" Shintaro startles a bit, but complies and walks over to the happy family. He peeks over Aka's blanket before the little one gurgles in glee upon meeting her new uncle. Takane giggles with her, "Yup, your new uncle's really funny-looking, isn't he? But he's a good guy." Takane looks up at him then, pure adoration in her eyes. "Uncle Shintaro, we can count on you to take care of Aka too, right?" And Shintaro doesn't even think twice before he blurts out a loud "Yes!"
The Twelfth Year
Shintaro is twenty-six and still without his past memories. But he never feels lost with the support of his friends and family. He's still writing, always writing, still keeping her up to date with current events.
The Thirteenth Year
Shintaro is twenty-seven and still hasn't regained his memories. But he still knows that he loves her and continues to write to her each and everyday. Aka is now two and is shaping up to be just as spunky as her mother. Shintaro chuckles, recalling the drawing Aka had given him the last time he'd visited. Apparently, she'd also gained her father's artistic ability because Shintaro certainly wasn't that good at coloring inside the lines when he was her age. He tells her more, about his mother, his sister, running into her siblings and being subsequently interrogated by the shortest of the trio. But he thinks that both her and her parents raised them well. He finishes the last sentence, and like many times before, sends the letter away without expecting any reply.
The Fourteenth Year
Shintaro is twenty-eight and finally regains his memory. But instead of celebrating, he breaks down crying in the middle of his room because he remembers, he finally remembers.
Ayano died fourteen years ago.
The Fifteenth Year
Shintaro is twenty-nine and still writing. He's hurriedly finishing his letter for the day before Takane storms into the room. "What are you still doing here?! Your sister's going to be walking down the aisle any minute!" Takane drags Shintaro out of the room and down a corridor where his sister is waiting in a beautiful white dress. "You're late, onii-chan!" She puffs her cheeks, putting both hands on her hips. Takane laughs and pushes him forward, "Yeah, well, your brother had a little something to finish," she says, gesturing to the slightly crumpled letter in Shintaro's hand. Momo's eyes widen in realization and her smile softens, "I see. Were you telling her about my wedding?" Shintaro unfolds the paper and mimics his sister's smile, "Yeah. It's a big day, you know? And I thought she'd like to know." Momo nods in understanding before offering her arm to her brother. "Thanks, Takane-san. For bringing him here." Takane laughs, shaking her head, "You can thank me all you want later. For now, I think there's someone that's really eager to see you."
As Shintaro walks his sister down the aisle, he sees Hibiya stiffen up by the altar. His best man, or rather, best woman, elbows him softly before flashing him an assuring smile. He hears a soft "Thanks, Hiyori" as he approaches them. Momo's prediction from four years ago had been spot on, and Hibiya had grown up to be tall and handsome, taller than Shintaro himself. Once they reach the altar, Momo plants a soft kiss on her brother's cheek before turning to stand by her husband-to-be. Shintaro turns as well and takes a seat next to his mother, trying his best to hold back the tears that threaten to flood down his face.
After the wedding, he's greeted once again by Ayano's father, who was apparently Momo's high school teacher as well. "Your sister told me that you've got all of your memories back." Shintaro nods, reflexively reaching for the letter in his suit pocket. "You know, I'm sure she's read all of them. I don't know where she is now, but if she could, I'm sure she would have written you a reply each and everyday." He turns then, ready to greet the newly wed couple. Shintaro smiles, taking the letter out and mentally taking note to buy some more stamps on his way home.
The Sixteenth Year
Shintaro is thirty when he finally receives a reply. After waking up on the morning of the sixteenth year of writing letters, he finds a scrap of paper next to his pillow. On it were but two words: "Thank You."
*Ikeneet - a combination of the words "ikemen", literally meaning good-looking guy, and "NEET", a person that's not in employment and never leaves their home.
*Aka - literally means red
If it wasn't made obvious enough, this story is set in an AU where Ayano's death successfully halted the snake(Kuroha)'s plans and Kenjirou never killed Takane and Haruka.