Note: That green jumpsuit. Enough said.
Warnings: None
Pairings: None
Characters: Might Gai/Guy/G'eye/however it's supposed to be spelled nowadays. And Kakashi.
During Gai's childhood, his wardrobe consisted of rather nondescript shirts, pants, and boots customarily worn by ninja. To understand his sudden and unusual shift in his taste of clothing, it is necessary to look back into his earlier years.
It started from the moment Kakashi beat Gai in their first Academy sparring match, when a beautiful rivalry was born.
Well, "beautiful" for Gai meant "irritating" for Kakashi, while "rivalry" for Gai meant "nuisance" for Kakashi, but for all intents and purposes, it was essentially a beautiful rivalry. Every few weeks or so, Gai would either propose a challenge to his rival, or force his rival to propose a challenge. The matches of strength, skill, and stupidity soon became legendary among the entire Leaf Academy.
Gai put all his heart and soul into the rivalry, going above and beyond each challenge his rival came up with. If Kakashi decided to race to the top of the Hokage monuments, Gai proclaimed that he would race to the top, and back, and back to the top fifty-seven times in a row (often in a voice loud enough to deafen passing birds, and with a shining smile that would blind them, too).
Kakashi, on the other hand, tried his best to go along with whatever his "rival" said, in a fashion that wouldn't embarrass him in front of the entire ninja population of the Leaf. Thus, if Gai challenged Kakashi to run three-hundred and twelve laps around the school in his underwear, Kakashi would ever-so-politely decline the challenge (often in a voice icy enough to freeze the now-decrepit birds into solid chunks of ice).
This "beautiful rivalry" went on in such a fashion for years. Gai continued to see it as a source of joy, while Kakashi increasingly saw it as a form of mental and physical torture that the ANBU should look into using on the prisoners of war.
Years went by, and the ages of the two rivals cleared the double-digits. Not long after Kakashi's eleventh birthday, an acquaintance from his academy years (a boy who was a year ahead of Kakashi and went by the name of Asuma Sarutobi) approached him with a wide, evil grin and a proposal. Apparently, Asuma had been beaten by Gai in a sparring match, and was looking for an underhand method of getting back at Gai.
The next time it was Kakashi's turn to suggest the challenge, he pulled a bright-green jumpsuit from behind his back and held it up in clear view of his rival.
"What in the name of our rivalry is that? Some kind of odd flag?" asked Gai quizzically.
"No, Gai. This is an amazing piece of ninja gear. Not only does it make you stronger and more youthful as you train in it, it helps you blend into the greenery and sneak up on your enemies, so that you may attack them when they least expect it."
"I see!" Gai's eyes went wide with admiration. He then frowned, "So what does it have to do with your challenge?"
"My challenge to you, Gai, is to wear this for the rest of this week to show everyone just how much you care about being a great ninja. By wearing this, you will show everyone how important it is that they take advantage of the power of their youth. A ninja, after all, must encourage excellence in his comrades," said Kakashi in a stony tone of voice that betrayed none of the humor he felt at the situation. He had no doubt that Gai would accept this challenge, "inspired" by their dear friend Asuma.
Gai's eyes gleamed with excitement as he snatched the suit from Kakashi's hands.
"I will go above and beyond this challenge, my eternal rival! I will wear this suit for the rest of my life, and I will vocally encourage everyone to believe in the power of youth!"
Kakashi's eyebrows rose a little at the proclamation, but he took it in stride. Asuma would have to pay up in full for this.
Gai was thereafter convinced that wearing a stretchy green jumpsuit was the epitome of his achievements.
For the first time in his life, Kakashi found the rivalry beneficial. Using the money that Asuma gave him, he went to the local bookstore and splurged.
Note: If things really went this way, I'll bet that Kakashi is sorry he took Asuma's bribe, don't you think?