I best explain the background for this story. See, the animation of Diabolik Lovers state that Ayato is the older of the triplets, however the wikia says he is the younger (so does MyAnimeList and probably other official sources too) Now, here is a theory of how this could be. Furthermore, I personally find it very unlikely that they are born on three different dates. Two is realistic because the births can be right around midnight, but three dates is a little much in my opinion.
I'm sorry if something is not how it is supposed to be. I only watched the anime for the sake of writing this, I only got in the characterizations that I picked up from the anime and the wiki. Also sorry if some of the actions are a bit OOC.
Ayato was lying on a couch in one of the many rooms in the house. He was really exhausted for some reason and didn't know what else to do. School had been a bother and he didn't feel like doing anything. Earlier today he had thought that maybe taking a sip of Yui's blood could help, but he really didn't want to drink from her today. The idea of trying to drink her blood while she struggled seemed so little appealing to him.
Thinking of that sweet blood, Ayato thought he could smell it somewhere nearby. His suspense was confirmed when the door into the room opened and Yui entered. A scent almost as sweet as her blood was around her, but it was different than her blood. It obviously wasn't blood.
"Oh," she said once she saw him lying there. "Ayato?"
Ayato turned, lethargically slowly, and faced her. "What do you want, Chichinashi?" he asked.
"Oh, I-" Yui instinctively took a step back. "I just…" she trailed off.
Ayato let his gaze trail up and down Yui's body. She was holding a tray between her hands. On it were some freshly baked cookies. He knew she could cook. She had made him takoyaki countless times, but he had never thought she would go bake something of her own intuition.
"Why did you make those?" he asked, his gaze locked on the cookies on the tray.
Yui looked down at the cookies. "Oh these?" she asked and looked up at him. She blinked a couple of times, supposedly to make herself appear more innocent. "Today is my birthday, so I wanted to make something a bit special," she said.
"Your birthday, huh?" Ayato got a thoughtful look. "So do you expect us to do something special on our birthdays too, Chichinashi?" he asked.
Yui blinked in confusion. "No, normally someone makes something for you on your birthday," she said.
Ayato leaned back. "So, you expected us to do something for you and then you had to do it on your own, honestly, how selfish can you be?" He smiled. "Well, if someone has to make me something special for me on my birthday, I order you to make me some perfect takoyaki," he said.
Yui stood still, taking the time to let it sink in. "But I don't even know when your birthday is," she said hesitantly.
Ayato got up and walked up as he stood next to the flat chested girl. "So you say you will make me takoyaki on my birthday if you know what day it is?" he asked teasingly.
Yui turned her head in an instant, her movement was so sudden she nearly dropped the cookies. "I-I never said that, Ayato!" she exclaimed.
"Okay, I'll find out and then I'll tell you the date and you will do it," Ayato said, ignoring Yui's protest as he picked one of the cookies from the tray and left the room in search of someone who knew his date of birth, because he sure didn't.
While eating the cookie Ayato made his way down the hallway in search for whoever of his brothers could have a clue about when he was born. He wondered who to ask too.
Shuu probably won't even give him an answer in the first place, he would just say he didn't know and let that be it. He couldn't count on Reiji either. There was no way that would happen. Subaru was too young to know of that, he shouldn't even begin to count on either of those three since they didn't grow up together in the first place. This only left Laito and Kanato.
Kanato won't take it seriously. He would ask his teddy but that would be the most of action he took in the matter and then conclude that he didn't know. Then there was Laito, the only person who would probably take him seriously and try to come up with an answer if he was lucky.
So Ayato decided to search for Laito to figure out the answer to his question. Meanwhile more ideas started taking shape in his head.
If he knew his date of birth he could not alone have Yui make him takoyaki on that day. He could also demand complete ownership of her body for an entire 24 hours. He could demand to have her all to himself and see what would happen about that.
Ayato reached the room where the piano was located. He could faintly hear someone playing behind the doors. There was no doubt that was Laito playing in there. Ayato therefore teleported himself in there, without bothering to knock or open the door, which would also have been way out of character for him.
"What do you want?" Laito had stopped playing the moment Ayato appeared next to him.
"I want to know when my birthday is," Ayato demanded.
"How would I know?" Laito asked looking up at Ayato in such a way that Ayato couldn't see his eyes because of the hat he was wearing.
"So you're saying you don't know?" Ayato turned his back to Laito.
"I never said that," Laito stood up "I know that Kanato was born on March 21st and I was born on March 20th, you were probably born on the same day as me, on the 20th," he said.
Ayato turned as he was facing Laito again. He had a frown on his face. All his ideas and plans for what he would do to Yui on his birthday were now completely ruined because of Laito's words. It couldn't be right. Ayato refused to share his birthday with someone else.
"You're lying," he determined.
"Why would I?" Laito asked. "It is really that simple if you stop and think about it,"
"Because you're trying to ruin my plan," Ayato spat.
Laito rolled his eyes. "What plan?" he asked dryly.
"You already know," Ayato hissed.
Laito sighed. Wondering why his brother had to be so hopeless. "I don't. What is this plan of yours?" he asked.
Ayato sighed and explained the entire birthday-idea that Yui had given him and exactly how this ruined his plan. He didn't notice, but as he spoke, a smirk appeared on Laito's face.
"You're right," he said after Ayato had stopped talking. "You were actually born on March 22nd. I was born on March 20th and Kanato was born on March 21st," he finished.
Ayato smiled. "Is that so, then I better go tell Chichinashi my date of birth," he turned on his heel, preparing to teleport back to where he had last seen Yui.
"Don't forget to tell Bitch-chan mine as well," Laito called before Ayato could leave.
Ayato teleported without a word to respond and landed in the room where Yui had been when he left her. She had disappeared now though. He wondered where she had gone off to.
"You can tell her yourself," he mumbled as a response to Laito's words. He sure didn't intend to do the other's dirty work. He didn't owe Laito anything.
Back in the room with the piano Laito was laughing like a madman. He couldn't believe his brother had fallen for the lie he had told. If he could keep this prank going then Ayato would believe Laito was the eldest of the triplets for an eternity.
There was only one issue with his plan. If all of them told Yui when their birthday was, would she figure out Laito's lie? If she did, he had no doubt she would inform Ayato. It wouldn't be too long before they were close enough for that. But until then…
Laito laughed as he shook his head. "There is no way Bitch-chan will put the pieces together," he said confidently. Then he too left the room to find something better to do.
So that's that. I hope you at least found it a bit funny. I'll say I did enjoy writing it regardless of everything else. I am not expecting much response though, since I gave off that little talk in the beginning and since I'm new to this fandom (and probably won't return)
Bare in mind, that if this is not the way they talk in the sub, it is because I only watched the dub of the anime. I picked up their pet-names for Yui on the wiki.
I currently write fro the anime Free! a lot, and also write some for Kuroko no Basuke, and I'm working on stuff for Owari no Seraph. If you like any of those, be sure to check out whatever else I'm doing.
Jackie