In the hands of someone good at writing this concept would probably be amazing. Instead, you have this. Enjoy? ( or maybe you shouldn't, considering the subject matter )
WARNING. DARK FIC. CHARACTER DEATH. PLEASE DON'T COMPLAIN
"Black Star-!" Tsubaki bit back the rest of her warning and could only watch helplessly as her meister charged in without her again. It didn't matter how many times she told him to think before he acted, Black Star was Black Star and he was determined not to change.
His opponent, Professor Stein, grinned. The assassin pulled him arm back and dropped his shoulder, his stance broadcasting his trademark 'Big Wave'. No, Black Star-! The ninja weapon groaned and looked away, already knowing the outcome of the fight. He can match your soul wavelength! Why can't you remember that? Why can't you ever learn?
Stein drew back and shifted to the side, allowing the assassin's attack to pass right by him. This left Black Star wide open and the doctor grinned malicious before slamming his hand forward, palm open as it smashed into Black Star's face. His fingers tightened against his forehead and a wave of electrical energy was pumped straight into the blue-haired meister's soul. His legs crumpled beneath him when he was released and the teen fell to his knees.
"Black Star!" She ran to him, kneeling down and catching him before his head could smash against the ground. He grinned in response, his smile caked in soot and blood.
"Don't worry Tsubaki," he got out. As she stared he lifted his hand to give her a thumbs up. "I'll be okay."
"Tsubaki! Enchanted sword mode!"
"But you still can't handle that!" She looked over at him from the reflection on her scythe blade. Black Star's focus was on the kishin egg before them, an exceedingly unpleasant creature that was reminiscent of the offspring of a bull and some sort of slime monster, but he still managed to give her a crooked smile in response to her worry. Tsubaki grit her teeth, refusing to play a part in damaging her meister's soul any further. "We haven't even been able to hold it for more than ten seconds in training. You're not ready!"
"Tsubaki!" She flinched at his tone and flinched again when Black Star threw himself to the side to avoid a charge from the monstrosity. As the beast ran past he whipped one of the ends of the chain out, curling it around its back leg; when it reached the end of the slack Tsubaki felt an unpleasant vibration run through her body as it pulled her taunt before slamming against the ground. "Do it now and we can end this!"
Against her better judgment Tsubaki nodded and started to shift, shuddering when she could feel the darkness coiling around his soul. Black Star was unaware – or at least he acted that way – of the danger he was in and he grinned as the shadows spread out in across his face. "Don't worry Tsubaki," he said lightly. "I'll be okay."
"Oh Black Star…" She could barely muster up the energy to care when she saw the grades posted from the latest exam. Maka and Ox were tied for first, of course, and Kid wasn't far behind. Tsubaki was seventeenth, Soul was twenty-four… and Black Star was all the way down at the bottom in last place. Again. "We spent so much time studying for this. How could you get a zero again?"
"It's the stupid professor's fault. They're asking all the wrong questions." He scowled, hands jammed into his pockets as he kicked as the ground. "Why am I supposed to care about what some dead guy did so many years ago? All that matters right now is just what I'm going to do."
"Are you trying to fail?" Tsubaki asked, unable to keep the accusatory tone from her voice. Black Star paused before looking up and over at her, appearing genuinely shocked at her frustration. "We spend hours going over everything and you don't even seem worried at all that you're failing all your classes. Are you just not taking this seriously? Do you think this is just a joke?"
"Hey, I know it's not a joke," he responded, though his tone was far too mild for her. "I want to do good… but it really doesn't matter in the end. Whatever I end up doing is going to involve me making sure the entire world knows my name. You don't need to be smart to do that."
"Black Star…" She felt like something was snapping inside of her. Tsubaki looked down at the ground, the frustration and angry hitting a breaking point. He doesn't understand, why can't he ever just understand! A hand suddenly slapped against her back and the ninja weapon's eyes widened as Black Star stood by her. He grinned rather cheekily as he glanced over at her and she knew what he was going to say before he even started to speak.
"Don't worry Tsubaki. I'll be okay."
"Don't worry Black Star."
He had seen her coming, had seen the pillow, but his sleep-addled mind wasn't able to add the two together fast enough. She was sure he had also thought she was joking – this was Tsubaki – and it wasn't until Black Star had wasted precious seconds that he realized she wasn't letting up.
"Please don't worry."
Tsubaki crooned softly as she bore her weight down on him. Black Star thrashed and flailed and one hand caught the cord of a lamp. He dragged it free and smashed it against her head, causing a moment of blindness for the weapon, but she refused to let up. Even when he reached out and grabbed her arm, clawing at her wrist, even when he started to go limp and she could feel his soul weakening. The ticking seconds turned into minutes and she stayed there until she was sure he was done moving.
As she rocked back onto her heels Tsubaki lifted the pillow. He looked peaceful and, as long as she didn't look too closely, she was sure she could convince herself he was just sleeping. And really, it was for the best. This way he would have to listen to her.
"… it'll all be okay."