It started the first time Ryohei punched Aoba in the face.
Aoba's glasses had flown off of his face and skittered to some unknown corner of the room, and he'd blinked and thrown a punch that had gone so wild even Ryohei figured out that he couldn't see.
"WHOA, HEY, I EXTREMELY CAN'T FIGHT A BLIND PERSON, HERE WE SHOULD GO FIND YOUR GLASSE-"
The transfer student suddenly reached out, groped for Ryohei's head, and, once he'd centered his grip to cradle both cheeks in his palms, drew one hand back and landed a hit square in the nose this time.
"YOU SHOULD NEVER UNDERESTIMATE A PERSON FOR THEIR PHYSICAL WEAKNESSES! I COULD STILL TAKE YOU ON WITH A LEG TORN OFF!"
Ryohei, while attempting to stem the flow of blood, briefly wondered what the odd fluttery feeling in his stomach was, and then brushed it off and threw a left hook to his opponent's gut.
He never did notice that odd reddish taint to Aoba's face.
Aoba was right in that he was smarter than Ryohei (though he wasn't necessarily right in other areas), so naturally he was the first to honestly figure it out.
The first time he'd felt jealous, it was of Gokudera when he and Ryohei had started another session of yelling their throats raw at one another. This was easy to pass off as anything else, really; he could have been jealous because it looked like an engaging argument, or he could have been feeling contempt for how childish their insults were.
Except then he felt the same feeling, only moreso, when Ryohei expressed excitement at the idea of fighting Lussuria again, and he decided to stop lying to himself.
Ryohei possibly could have taken years longer to figure it out himself if Kyoko hadn't pointed out how much time they were spending near each other.
"Onii-chan must get along very well with Aoba-kun."
Which, naturally, he'd disagreed and argued with until he really started thinking on it and realized it was kind of true.
Aoba was rather hellbent on convincing himself that it was going to be no more than a passing interest, which left Ryohei as both the more forward of the two and the one who pretty much had to make the first move.
Not that he had the logical capacity to figure this out himself. But he asked Kyoko how most girls asked out their crushes anyway. And then he took the first idea she gave and got the heck out of there, because even if it was his one-and-only little sister, it was still a damn awkward topic to be questioned on.
... which. Didn't necessarily mean it was a good first idea.
This was how Ryohei ended up trying to write a love letter.
He gave it his all, he really did. He spent hours, scattered across different days but still collectively a plentiful amount of time, crouched over pieces of paper trying to come up with a way to properly convey his feelings in a way that didn't include insults, overuse of the word EXTREME, or anything that was just completely embarrassing.
... except that everything he came up with that didn't include one or both of the first two looked like it had five different examples of the second in as many sentences.
So he didn't actually succeed, which left him to go ask Kyoko for a new idea.
This time, Kyoko managed to corner him while he was still putting out the fire and giving up on the chocolate idea. (Not that anyone told him it counted as giving up, since then he'd just keep trying and would destroy the kitchen further.)
"You shouldn't have asked me what a girl does if you wanted to ask someone out, silly!"
So then she suggested flowers.
To be specific, she'd suggested roses, and Ryohei had been fine with this idea at first because the thorns were pretty epic, but then when he'd gotten to the flower shop he'd caught sight of the sunflowers and...
So, naturally, he bought about fifteen of them. Full-sized ones.
The next day he carried them to school, pretty much shoved them in Aoba's face and asked the bespectacled boxer out on a date for all the school to hear (which they probably did).
Aoba replied by squawking, stuttering out something incoherent, and punching Ryohei in the face again to hide the fact that his face was about as red as the First Storm's hair.
Which, of course, meant yes.