Tale of the Setting Sun.

Chūnin Exams VII

"Yes, sensei," Naruto said, his tone now touched by a slight flavor of impatience.

"Alright then." Kakashi took a deep breath. Might as well get it over with from the get-go. "You are a jinchūriki." At these words, he noted that Naruto's face remained unchanged, but could sense his body tensing up. "That means that you are the host of a tailed-beast. In your case, during its invasion of Konoha, the Nine-tails - Kyūbi - was sealed within you when you were a newborn baby."

"The Kyūbi wasn't destroyed?" Naruto asked, raising a fist to clear his throat. His face was otherwise, curiously devoid of emotion. "I thought the Fourth Hokage defeated it."

"Tailed-beasts aren't so simply defeated, Naruto," said Kakashi. "They're mammoth living forms of chakra - the best we can do is seal them away. And most villages take advantage of this by sealing them inside another living vessel - a human."

"Was it a complete seal - is that even possible? How come it's never gotten out?" Naruto frowned. "It has, hasn't it?"

Kakashi nodded. "Correct. However, from what I know, there has been only one incident so far in which the Kyūbi managed to temporarily escape its seal. It happened when you were six - or actually, the day before your sixth birthday. Some foreign ninja somehow managed to get intel on you and infiltrated Konoha. In the incident, you lost control and managed to kill them all."

"But I don't remember - "

"There are gaps in your memory, Naruto. Ever since that day, for five years now, there has been a seal on some of your memories." It was curious and yet disheartening to observe the careful look of calm beginning to fall apart on the genin's face, like cracks in a sheet of ice that had frozen over a raging river. Before he could be swept away in its current, Kakashi hurriedly pressed on. "It is a type of forbidden fūinjutsu called the Genkōkin Fūin (Taboo Seal). It was invented by the Fourth Hokage, and was popularly used during the Third Shinobi World War - it works by making a word or idea 'taboo' in someone's mind. Any time they begin thinking about it, the seal immediately sucks the memory away. People whose mental resilience is strong enough tend to unconsciously fight this, and suffer tremendous headaches as a result. That's the reason why you've been falling unconscious. Your mind has been fighting with the seal, trying to hold on to your memories. As a temporary measure, I've put a block on the seal for now, which is why, as I tell you this, you aren't falling unconscious."

Naruto's hands fell by his side. A long, terse silence entangled itself around them. Looking at the mixture of feelings playing out across his student's normally stoic face, Kakashi had to hold himself back from sighing. Ninja lived harsh, often unjust lives, and it was something that he had come to accept a long time ago as a child himself. But something about the way Naruto's face looked like just now brought back memories of those grey, rain-drenched days he'd spent in the reflection of the cold memorial stone.

"Why me?" Naruto finally let out.

"The Fourth Hokage chose you," Kakashi replied.

"Why?" Naruto repeated, shaking his head. Kakashi hesitated. "Please, sensei."

"Because you were his son," he said quietly.

"What?" Naruto blurted out, looking at Kakashi in naked disbelief. "But that's..."

"Your father was Namikaze Minato, the Fourth Hokage," Kakashi pressed on. "He was my teacher, and a legendary shinobi renowned as Konoha's Yellow Flash. He died a hero, saving the village by sealing the Kyūbi inside his own child - you."

Naruto didn't respond, and as Kakashi watched the genin's eyes slowly turn dull, the weight on his back seemed to grow heavier and heavier. He wondered if he had made a mistake - not about putting a stop to the memory seal, but about the timing of it all. Perhaps he should have waited for a more optimal time before revealing the truth to the boy? But with the memory seal quarantined off, Naruto would have woken up immediately knowing that something was off, and it would have been damaging to their tentative trust if Kakashi had continued denying him the truth of everything.