This is inspired by hetaliatimes on tumblr. They came up with the whole premise and idea.
There was a period of time, somewhere between the end of kindergarten and the second year of elementary school, that lay deeply buried in the minds of everyone who knew Tsuna.
Tsuna didn't exactly win the name lottery with his name. Apart from the misfortune of having ancient name of Tsunayoshi, he was cursed with one more thing:
Having Celio, an Italian name, as his second name.
This brought his name to fourteen syllables: Sawada Tsunayoshi Celio.
He never liked his full name. Tsunayoshi was a mouthful and Celio often got mispronounced as Cherio. Soon, he found a compromise in the name 'Tsuna', and stuck with it.
If only the people around him could do the same.
"Why don't you like the name Celio?" his mother asked him one day, when he reacted rather negatively to her addressing him as 'Celio' instead of 'Tsu-kun'.
"I just don't like it," was Tsuna's reply. Nana pouted and folded her arms.
"Well, it's a name chosen by Papa! You should like it!" she insisted. Tsuna didn't answer, not wanting to tell his mother that the knowledge only made him hate his name even more.
"I still don't like it," he eventually answered. Nana sighed in defeat and dropped the subject.
Thankfully, she never called him Celio again.
The problem with having an Italian second name was that everyone around you immediately paid more attention to you. And they tended to use your full name for the hell of it.
"Sawada Tsunayoshi Celio, are you a half?" they would ask.
"Not really," Tsuna would reply. "I just have some Italian blood from my father's side. Also, please call me Tsuna."
"That's so cool, Sawada Tsunayoshi Celio. You have foreigner blood in you!" those people would marvel, totally disregarding his words.
Tsuna would just grit his teeth and bear with it. After all, what else could he do?
Anyone would tell you that suppressing your emotions isn't the best way to cope. It would just build up more and more and more and one day you wouldn't be able to contain it anymore.
Step one: Bottle up your emotions.
Step two: MOLOTOV COCKTAIL
It happened one bright and sunny day. It had been a rather peaceful morning, the birds were singing their melodious bird song, entertaining the world and bringing a smile to the faces of most when suddenly, out of one of the classrooms came...
"I'LL BURN EVERY SINGLE ONE OF YOU TO DEATH!"
What happened next was so devastating and disturbing that everyone immediately forgot about it once it happened, along with the trigger to the event.
(Well, everyone but Hibari Kyoya, who had coincidentally been walking past nearby, forgot about it. He would fall in love with that phrase and he would later modify the "burn" to "bite" to suit his personality. He would search high and low for the wonderful person, his role model, who had uttered those words. Alas, he wasn't able to find them for everyone who had witnessed the event had blocked it out of their minds.)
Everyone repressed the knowledge that Tsuna's full name was Sawada Tsunayoshi Celio, substituting it with just Sawada Tsunayoshi instead.
After one year in elementary school, Tsuna would get saddled with the name "Dame-Tsuna", but that was alright. It was infinitely better than freaking Sawada Tsunayoshi Celio.
He was content, sure in the fact that he never had to hear the name again.
And then, when he was thirteen, a baby hitman intruded into his house.
"Sawada Tsunayoshi Celio," the hitman said. "I am here to train you to become a Mafia boss."
"Just call me Tsuna. Please." Tsuna wasn't above begging.
"Absolutely not," Reborn replied, smirking with sadistic amusement. "Celio is a better name for a Mafia boss than Tsuna."
Sensing that he couldn't talk Reborn out of it, Tsuna despaired.
"Sawada Tsunayoshi Celio, right?" Dino asked. Tsuna twitched at the name.
"Just Tsuna's fine, Dino-san," Tsuna said. Dino nodded.
"Right, Celio…" Tsuna twitched again. A glint from the corner of his eye caught his attention and he turned to see Reborn polishing his gun. Tsuna inwardly screamed in frustration as he kept a mild expression on his face.
He hated the name Celio.
"Kufufu, you must be the Vongola Decimo, Sawada Tsunayoshi Celio," Mukuro commented. Tsuna scowled.
"It's Tsuna," he hissed. Mukuro smirked.
"I'll refer to you Mafia scum however I like, Vongola Decimo Celio."
When Tsuna fought Mukuro, he took great pleasure in beating him down.
Byakuran kept referring to Tsuna as 'Sawada Tsunayoshi Celio'. If he was feeling nice, he'd just say 'Celio'.
From the start, Tsuna knew he wouldn't get along with the white haired bastard.
If anyone had been looking closely at Tsuna when the boy fired his X-Burner at Byakuran, they would have seen a particularly bloodthirsty grin.
While Yamamoto hadn't seen Tsuna's expression, he still shuddered at the sight, the words "I'll burn every single one of you to death" echoing in his mind for some odd reason.
The Vongola Nono often called Tsuna 'Celio' as well. Tsuna wondered what it was with Italian people and his name. Was 'Tsuna' hard to pronounce or something?
"It's Tsuna," the boy constantly corrected. To his despair, the ninth never took the hint. Either that, or he didn't care.
If the Vongola Nono hadn't been an old man, and hadn't been his surrogate grandfather, Tsuna would have long shouted at him about it.
It really should be obvious that Tsuna hated the name Celio. Why else would he violently flinch whenever someone referred to him by that name? Why couldn't anyone take the damn hint?
The phone rang and Tsuna absentmindedly picked it up.
"Ah, Celio!" Dino said. "I—"
"Sorry, there's no one named Celio here," Tsuna immediately interjected before slamming down the phone.
He later had to call Dino back and apologise while Reborn held a gun to his head.
It figured that when he finally got on 'first' name basis with the members of the Varia (and that happened sometime after the whole deal with Checkface), they would call him 'Celio'.
"Tsuna," he stressed, trying to correct them in vain.
"Celio," they would say, dismissing his words. Tsuna wanted to cry. Or kill someone.
All good things come to an end.
"Celio, Celio, Celio, Celio."
Similarly, Tsuna's patience came to an end.
"ALL OF YOU WILL BE BURNT TO DEATH!"
The next time Dino called, Tsuna braced himself.
"Hey Tsuna, how are you?"
And he got a very pleasant surprise.
Apparently, Tsuna's outburst had been enough motivation to get everyone to call him Tsuna or Tsunayoshi. Well, everyone but…
"Dame-Celio!" Reborn called out, making Tsuna cringe rather violently.
"What is it, Reborn?" he asked after recollecting himself. Reborn was standing in front of him with a rather strange expression on his face.
"Why do you not like the name?" Reborn asked. Tsuna shrugged, shoving his hands into his pockets and scowling at the floor.
"I just don't like it. It makes everyone form strange impressions of me. My dad chose that name. Choose whichever reason you prefer," he said. "Not like you'd care anyway," he added under his breath. There was a tense silence between them for a while before Reborn kicked him in the head.
"Don't talk rudely to your superiors, Dame-Tsuna," Reborn said. Tsuna's entire body jerked at the name, as he turned to stare at Reborn with wide, unbelieving eyes.
"R-Reborn?" he stuttered out. Reborn smirked at him.
"Mafia bosses don't stutter, Dame-Tsuna. Looks like I'll have to train you again."
"Hiiiee!" Tsuna backed away, feigning terror at Reborn's threat.
Reborn pretended not to notice the large, grateful smile on Tsuna's face.
I don't really like it when people don't call people by their preferred names. It's like, they don't respect you as a person. They don't care about you.
If someone calls you by a name you really don't like, leave them. They are literal trash and you shouldn't concern yourself with them. Either that or BURN THEM. Whichever won't get you arrested.
Celio comes from Caelius (which comes from caelum) which means heaven. I would have chosen Cielo but that's overused.