Last two chapters of Bleach gave me this idea, although all characters are their pre-timeskip versions.
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"I don't want to!" shouted the young white-haired boy of with a scowl.
"Aw, why not?" questioned a boy with greenish black hair with a confused look.
"Because I don't want to!"
"Ah c'mon, Snowflake. Just show us your little trick," said a boy of thirteen with a lizard-like grin.
The "Snowflake" glared at the older boy for the nickname. How did they even find out about his secret ability in the first place? He barely knew anything about his ability!
He also questioned why these three were here. One minute he was doing his paperwork and the next minute Naruto, Gon, and Koichi were here asking him to show them something he did want to!
"I don't get what the big deal is. Why don't you just show us your adult form, Toushirou?" questioned the blond fox boy.
Toushirou grit his teeth in frustration. "I don't like my adult form. That's why, Uzumaki! And it's Hitsugaya-taicho to you, Kasuchi!"
"Whatever you say, Snowflake," stated Koichi with a roll of the eyes, earning a glare from the chibi taicho. "But seriously, you might as well show us your transformation. I mean Gon here has a very similar ability."
The young hunter gave off a grin. "That's right! Using my Nen, I'm able to age myself and become stronger!"
"Man, I wish there was a jutsu that could let you become an adult," said Naruto with a pout.
"If there was a jutsu that could do that, Tsunade would make damn sure you'd never learn it," replied Koichi, as he looked over at his blond friend.
"Why must Baachan be so mean?!"
Toushirou let out a sigh of frustration. Why couldn't these three leave him in peace? He was about to tell them to get the hell out of his office when…
"Oh, Taicho! You tell me you were having friends over!"
The dragon child glared at Rangiku who was standing in the doorway with a smile. The one time he didn't want her in the office and here she was.
"Hi, Obachan!" exclaimed Naruto with his fox-like grin.
The busty blonde let out a few giggles as she replied to her "nephew". "Hello, Naruto! So what brings you boys here to visit my cute little taicho?"
"We wanted him to show us his adult form," replied Gon with a smile.
Rangiku tilted her head in confusion while Toushirou really wanted to just run away. He did not want Rangiku to know about this!
"Adult form? What do you mean?" questioned Rangiku as she glanced over at Toushirou.
"We asked Hyourinmaru exactly how strong Toushirou is and he told us of his Bankai's true power," answered Koichi as he pointed at the Zanpakutou spirit, who was sitting outside watching the moon.
'So that's how they found out! How could you, Hyourinmaru?!' thought Toushirou with a scowl. For a brief moment, he thought he could feel the ice dragon smile slightly.
"And Taicho's Bankai can turn him into an adult? Taicho, how come you never told me about this?!"
"Because it's none of your business!" exclaimed Toushirou as he glared at the blonde. He then glanced away with a look of embarrassment. "And…I kind of…didn't know of it myself."
"Well now you know, so I say you and Gon should get into your adult forms and see who is stronger!" exclaimed the lizard boy with a grin.
"I'm game!" exclaimed Gon as he wound up his arm, ready for a spar.
"Well that will have to wait until the morning, since it is getting pretty late," said Rangiku. "Koichi, Naruto, and Gon, you three can stay here for the night and Naruto, I'll send Tsunade a message about where you will be so she doesn't worry."
"Okay, Obachan," replied Naruto.
"Matsumoto…" growled Toushirou. He did not like being treated like a child and therefore did not need to be told when to go to bed.
"Don't take that tone with me, Taicho," stated Rangiku as she gave the boy a narrowed look. "You are still growing and growing boys need their sleep, so off to bed with you!"
The dragon child let out another growl as Rangiku herded the four children towards the bedrooms.
"Kehkehkeh, you should have known better than to argue with Rangiku when she gets into her Mother Cat mode, Snowflake," laughed Koichi.
"Shut up, Kasuchi!"
Koichi let out one last laugh as he, Naruto, and Gon took some of the spare guest rooms and turned in for the night. Toushirou pinched the bridge of his nose in frustration before getting ready for bed. He could only hope that when morning came around that the others will have forgotten about his ability to age and that spar.
Knowing his luck though, it was unlikely.
FIN
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I'm actually glad that Toushirou's ability to become an adult is similar to Gon's in that he is a kid who can become an adult and not an adult stuck in a child's body.
Bleach is owned by Tite Kubo.
Naruto is owned by Masashi Kishimoto.
Hunter x Hunter is owned by Yoshihiro Togashi.
And I own Koichi.