Papa
It was sudden. If Reborn told you he'll tell you it was normal, but it was sudden. Reborn wondered why was be being called that. He only helped the kid from the streets and let him stay, besides he doesn't get attached to people.
So one morning out of nowhere, when Reborn returned from a mission, Tsuna called out, "Papa, Okaeri!"
Reborn was shocked at the name.
Tsuna had hugged his legs and brought his arms up, signaling that he wanted to be carried. Tsuna noticed Reborn's lack of reaction, at this point Reborn would hit him in the head for asking to be carried. "Papa?"
There it was again. Reborn broke from his shock and placed a hand on the boy's head. "What did you call me?"
Tsuna smiled. "Papa. Papa is Papa."
"Why?"
Tsuna tilted his head. He remembered he had a mom, but he never saw his dad. He once asked his mom about what a dad was. She explained it was someone who was always there to help their child. Well Reborn was home whenever he could be. She also told him dad's were cool and strong. Reborn had already fit the criteria for those as well. In Tsuna's mind Reborn was his dad.
"Papa is cool and strong," Tsuna said. "Papa is always with Tsuna, when he's home." Tsuna gave him a toothy grin. "So Reborn is Tsuna's Papa."
Reborn felt like facepalming. Who ever told this about dad were wrong. "Look, Tsuna I can't be your dad. You probably already have one."
"Does that mean Reborn can't be Tsuna's Papa," a sad frown appeared on Tsuna's face. "Eh… B-But T-Tsuna doesn't re...reme-reme…"
"Remember," Reborn filled in. He was going to have to teach this kid some words.
"Yeah, remember dad," Tsuna said with small tears prickling the corner of his eyes. "There was no dad at my house."
From what Reborn gather, Tsuna dad either dead, or was out working all the time. That meant Tsuna never had father figure and was now seeing him(Reborn) as that father figure. He looked at Tsuna's face, tears stains on his cheeks. He let out a sigh and ruffled Tsuna's hair. "Fine."
Tsuna head perked up. "Really!"
Reborn didn't say anything and picked up Tsuna, who let a out a laugh.