AN: My take on how Tanjiro's awakening as a demon might differ from Nezuko's and their first interaction with Giyu Tomioka

"Hold on," Nezuko panted. "I'll get you help, I swear!"

Her legs ached with each step. Every desperate inhale made her feel like her lungs were being stabbed with a thousand tiny needles, but despite the frigid temperature she was sweating. The only sound was that of her feet punching through the nearly frozen snow, as if the forest surrounding her home had been abandoned by all living things.

Nezuko forced herself to keep walking. Trudging uphill was so ardeus that all she could focus on was putting one foot in front of the other. She ignored the pain in her legs and in her arms, the ache of her lower back and of her lungs. She had to get Tanjiro to someone who could help.

Getting him all the way down to the village would be best, but she knew she could never make it that far. While she was no stranger to carrying heavy loads, usually they were in a basket or something she could easily tie to her back so she could walk upright. Tanjiro was taller than her and she'd not had time to waste trying to properly attach him to her back, instead being forced to keep her hands under his thighs and bend forward as she walked. Even with the adrenaline coursing through her she was tiring quickly. The best she could try for was Saburo's place. She didn't know how much medical knowledge he had, but she was sure he'd lend her his cart.

Tanjiro's weight shifted all at once and he hollered. Nezuko tried to compensate for the shift but lost her balance, stumbling a few steps backward as Tanjiro straightened up. Her next step backwards hit nothing at all and she lost her grip on him as she fell off the cliffside. She was too shocked to let out a yell of her own.

She was lucky for the snow that cushioned her fall. Had it been any other time of year she may have broken her back, and that thought alone made her shiver. It took her a second too long to realize she hadn't landed on Tanjiro, and she sat up to look for him. Nezuko was shocked to find him a dozen feet behind her and already standing up. She scrambled up as fast as she could and rushed over to him.

"Take it easy!" She cautioned, putting an arm around his shoulders. "Don't push yourself, I can car-"

Tanjiro let out an animalistic cry and there was the dull snap of teeth right next to her ear, so close that she could feel his hot breath. Nezuko jerked away in surprise and he grabbed at her shoulder, something small and sharp pricking through her clothes. She tried to pull away more and lost her balance, falling backward into the snow and pulling Tanjiro with her. She panicked and pushed up against Tanjiro's shoulders as hard as she could, trying to make sure he didn't irritate any of the wounds on his chest. It gave her a good look at his face and her stomach sank.

"Brother?!"

His face was set in an ugly snarl that bared sharp teeth he had never possessed before; veins along his forehead and trailing down his cheeks bulged prominently; his pupils had shrunk to slits and the dark red of his iris had changed into a light pink that rivaled her own; and there was nothing but anger behind those eyes. His scent had changed to something fuller and darker, something that made Nezuko want to run and hide.

Saburo's words from the night before came back to her and she tried to believe it wasn't true. She'd known Tanjiro her whole life and if he'd been any sort of demon before she would have noticed. Something must have happened when their family was attacked to make him like this, but try as she may she couldn't think of what it was. She tried to think up every story she'd ever heard but her arms were beginning to shake under his weight and she didn't know what to do.

Tanjiro started to growl and snapped his teeth together mindlessly. Her arms began to buckle as he suddenly grew on top of her, the added weight making it impossible for her to keep them straight any longer. Long fangs snapped within inches of her nose and for the first time she felt a wave of fear.

"Try your best, Tanjiro, please!" Nezuko begged. Tears stung her eyes, "You have to be in there! Come back to me, brother!"

The rumbling noise he was making ramped up and Nezuko's arms gave out. She squeezed her eyes shut, certain she was about to feel teeth tearing into her, but one long moment passed into the next with no pain. Something hot dripped down onto her chilled face and she opened her eyes to see Tanjiro that Tanjiro had embedded his teeth into his own arm and the blood was dripping down onto her face. Tears filled his eyes.

"Tanjiro, no!" Nezuko couldn't do anything but watch in horror, pinned under him as she was. "You're hurting yourself! Stop!"

Tanjiro scrambled off of her and Nezuko watched him teeter a few feet away and collapse at the bottom of a tree. He curled up as much as he could and put his arms over his head, uncaring of the blood that dripped freely from his arm. She sat up and then stopped moving when he flinched at the sound. He looked up at her and bared his teeth, snarled, and then bit a new wound into his arm. She was sure she heard something crack.

"Tanjiro…"

His attention turned from her to something just behind her, growl starting up again. The wind changed directions and she picked up the scent of another human. She turned her head to see a young man just behind her in the treeline. Black hair, dark blue eyes, a mismatched haori, and a sword at his side. She'd not heard him coming towards them and with how snow covered the ground she should have.

"Do you know what that is, girl?" The man asked her, though his gaze was trained on Tanjiro. The sharp scent of killing intent began to waft off of him, "A demon."

She knew. But the demon was her brother, so she didn't care. Her gaze fell on the man's sword and she read the kanji along the hilt, a promise to kill every demon. Saburo had mentioned some sort of great demon hunter that protected them all from the darkness that could emerge at night, but where had he been when her family was slaughtered?

"He's my brother," Nezuko stood up, keeping her gaze on this strange man the entire time. He looked pained when she said that Tanjiro was her family, but everything inside of her told her he was a threat to her brother and she couldn't trust what she picked up on his face. "He's not done nothin' wrong."

"There's a house a few miles away from here," the man went on as if he hadn't heard her at all. "The family inside was slaughtered. He did it."

"He didn't!" Nezuko couldn't help the way her voice trembled. From behind her Tanjiro's growling grew harsher, "That's our family! He was attacked alongside them! I don't know why he's like that!"

"If demon blood gets in an open wound you turn into a demon," the man responded.

She didn't care, "Leave us alone! He's not hurt anyone!"

"He tried to eat you."

"And where were you?!" Nezuko demanded, "You're one of those demon hunters, aren't you?! Where were you when my family died?! Help him turn back!"

"I'm sorry," blue eyes met hers and Nezuko knew he was going to kill Tanjiro. "There is no way for a demon to become a human again. I have to kill him before he loses control completely. I'll make sure he doesn't suffer."

Nezuko took a deep breath and nodded, letting the tears drip down her face freely. The man stared at her for a long second as if gauging her sincerity before he returned his attention to Tanjiro. She forced herself to stand still as he walked past her and then counted out five more steps before she pivoted and jumped at his back. It was easy enough to haul herself up when she'd taken him by surprise, wrapping both arms around his neck and squeezing as hard as she could.

"Run, Tanjiro!"

The man's hands scrabbled at her arms, trying to pull them away, but Nezuko fought it with all the strength she had left. He hissed out something that she couldn't discern and she clenched her eyes shut as he started moving them backwards, letting out a cry of pain when she was shoved hard into the trunk of a tree. She could hear the loud crunch of snow from ahead of them and was relieved at the thought that Tanjiro might be able to run in time.

The man's head slammed backwards into her nose, forcing her own head to thunk into the hard bark at her back. She felt something hot pour out of it and tasted iron on her tongue when she hissed in pain. Dulled from the double blow, her grip loosened enough that the man could get one hand in between her arms and his neck to give himself some breathing room.

"You stupid girl!" The man spat.

Loud footfalls came racing at them and Nezuko opened up her eyes to see that Tanjiro hadn't run after all. The man unsheathed his sword and held it straight out, but Tanjiro didn't act like he recognized the danger it presented. She watched in horror as her brother blundered forward, his own momentum pushing the sword right through where his heart was and causing him to yell out in pain. The man didn't seem bothered at all by Tanjiro pushing against the sword, arm kept straight to easily hold him at bay even with Nezuko clinging to his back.

But then Tanjiro grew taller again, the shoulders of his yukata ripping as he became more muscled. The man's arm buckled and Tanjiro pressed closer, the veins on his forehead becoming even more prominent as he grabbed the man by his shoulders and lifted him straight off the ground. Nezuko lost her grip on his neck and slid off his back, watching as her brother tossed the man into the air and away from them.

The man righted himself midair and landed on his feet. Tanjiro turned to face him, his taller body blocking her view of him completely until he crouched down some as he adopted a defensive stance in front of her. The end of the sword was sticking straight out from his back and the back of his yukata was so stained with blood that it was almost black. A wound like that would have felled an ordinary human, but he was no longer a human.

"Just leave us alone!" Nezuko pleaded. "I'll find a way to get him back to normal! Just walk away and he won't hurt you! I won't let him hurt anyone at all, I promise! So please-"

The man appeared next to them, so fast that she hadn't seen him move at all. His hand came up and he chopped at the side of Tanjiro's neck, her brother crumbling to the ground all at once. Nezuko sucked in a breath as she struggled to think of what to do as the man pulled his sword out of Tanjiro's chest. The sword turned to her, the point of it resting just underneath her chin, but he didn't do anything else with it. It was a warning to keep her still.

"If you listen to what I say then the two of you may survive this," the man started.

Nezuko began to cry in earnest when she realized she could no longer pick up the scent of killing intent from him. She was so relieved that her legs gave out underneath her, sheer exhaustion and relief making her head spin.