A/N: Mild spoilers for slightly more recent chapters. The ending is fictional, and by that I mean non-canon.
Naruto and Kyuubi had only one thing in common: neither had a choice in their relationship. Both were forced upon each other. It had been a pretty miserable existence at first. Kyuubi hated everyone and everything and was a demonic parasite in every way he could find. He stole Naruto's chakra and tried to take over him. Not exactly a good basis for a friendship. Naruto, for his part, didn't hate so much as he tried to deal with what he was given, even in his early years of being loathed by the village. Despite all of that, Naruto never hated Kyuubi. After all, it wasn't any more his fault than it was Naruto's, right?
This unspoken battle of wills was waged silently for years. The more Naruto grew, the more Naruto loved. Even the evil Nine Tails was not exempt from it. One day, Naruto visited his cage and spoke to him, leaving with a grin and "Hey Nine Tails, I wanna get rid of all that hate of yours one day!" And in typical Kyuubi style, he roared and cursed and spat, laughing bitterly into Naruto's face at his ridiculous naivety.
Madara stared them down, saying "You should both understand that misery… a little. You two have grown up as the hosts for tailed beasts, and you have endured the misery that comes with that role, have you not?" Naruto looked angrily at that masked "Madara" and the Nine Tails heard him say in earnest "Living with a tailed beast isn't such a bad thing." Kyuubi didn't realize his subsiding hatred until he felt something totally unfamiliar rise within him upon hearing that. His eyes widened and his once-black heart thumped.
When that stupid Eight Tales called him out on it, he felt something akin to (but much darker than!) his heart stuttering. Naturally, he turned away and ignored him, pretending to sleep. He was still full of hate. Full of it. Full to the brim, even. He had so much hate he didn't know what to do with it all. Except destroy everything. He was the Nine Tails, dammit. The Nine Tails didn't change for some stupid, naïve human child. Or did he?
Kyuubi laid his head on his paws and sighed through his nose, tails subtly twitching behind him. He couldn't explain his urge to help and protect this child, even without taking him over and becoming the one in control. It wasn't just a matter of saving himself anymore, he couldn't quite bear the feeling of stomach-churning fear when that boy was injured or in peril.
Kyuubi muttered angrily to himself and continued to fake sleeping. Inside, though, he wondered. "Is this why Naruto fights? Is this why he risks his life to save others?" He's never cared about anyone, he thought to himself. Is this what it's like? His tails continued their short waves.
The fight with the masked "Madara" wore on. Kyuubi could feel the strain in Naruto's muscles, the ache in his bones, the sweat on his skin. He could feel the tiredness, the determination, the need to fight through the pain. He could feel how low Naruto's chakra reserves were, even after the loan from the Nine Tails. He felt his claws dig into the floor, his tails swishing madly. He felt his pupils increase in size and a tightness in his chest. Naruto was hurt—bad. This masked "Madara" was too much for Naruto after having fought through his armies. Kyuubi felt his lip curl, exposing his teeth, and a low rumble crept from his throat.
As Naruto hit the ground with a sickening crack, Kyuubi felt his ears shoot straight back and with an enormous roar, brutal chakra exploded from Naruto. Naruto was back in the game, but with less of himself and more of the Nine Tails' fury. No, Nine Tails would not give up this boy without a fight. It's like he told him, as much as he hated being sealed in Naruto, he'd prefer it to that disgusting "Madara," who he hated the most.
At least, that's what he kept telling himself.
As he landed the final blow, panting hard and nearing the end of his chakra, he used most of what was left to heal Naruto as much as he could. It was enough to bring him back to consciousness. Naruto was dazed, but offered a genuine, dazzling smile. "Thanks, Kurama." In his mind, Naruto reached behind the bars and stroked Kyuubi's muzzle. The fox's ears went back in irritation, but he tolerated it. Naruto scratched behind the ear and Kyuubi was loathe to admit the feelings stirring in his chest at the obvious display of kindness and affection. "I'm not some damn house pet, brat." Naruto laughed sunnily, putting his hands behind his head. "No, I suppose you're not. Haha. We're a good team, Kyuubi. I told you that you couldn't hate forever." He winked at the fox, who snarled in response. "I never said I didn't hate you! I haven't lost an ounce of my hatred!" He snarled through the bars, chakra bubbling furiously around him. Naruto laughed and whispered softly with a smile, "You didn't have to say it." He walked off with a wave and Kurama sunk within his cage to hide his embarrassment.