Chapter Text
For as long as Senku could remember, he always had Tsukasa by his side.
The two first met in elementary school. Although Tsukasa was 2 years older than Senku, the older always visited Senku's classroom to hang out with him and waited for him in front of the classrooms when he started middle school.
Tsukasa didn't mind, no, he enjoyed Senku's obsession with science and protected him from others who didn't like a kid being smarter than them.
Senku was his light of hope after all.
That day when Mirai was admitted to hospital Senku declared, "There's nothing we can't fix with science. Just wait for me Tsukasa."
That was the moment Tsukasa decided that he'd do anything for Senku.
Later that week he quit school and started to fully focus on his matches. Since he wasn't 15 yet they weren't legal but it paid well enough for him to support Mirai and himself.
He was scared of Senku's reaction once he told him but all he got in return was a pinch to his cheek. "I knew you'd eventually do this. I just didn't expect it so soon." Tsukasa was stunned. "What do you mean you knew?"
Senku grinned. "You really should hide your smile as you beat up my bullies. It was no wonder you'd grow up to be a professional fighter." Tsukasa smirked. "Then you can practice hiding expressions with me so you don't watch me beating them with joy in your eyes clear as day." Senku blushed at that and pulled him by his shirt into his room and showed him a large folder full of his notes on mixed martial arts.
Tsukasa felt warm inside. Senku was preparing everything for him before he even knew. He took the smaller into a crushing hug. "Thanks." Senku now almost used to being hugged by him after many years, hugged him back. "You'll regret thanking me when I start tutoring you about all the school work you'll be skipping." He grumbled. Tsukasa smiled wider. "I knew you'd never let me off of school work."
Their high level of understanding each other helped a lot with the situation but it was still hard on Senku to see Tsukasa injured.
The first time it happened he couldn't stop trembling as he wrapped bandages around a very bruised Tsukasa. The brunet insisted that he could do it himself but Senku poked his bruise in reply. "I'm just surprised. You never got this hurt fighting off my bullies."
Tsukasa sighed. "You know this is different. No one I fight now is around my weight, height or even age range. I'll have to work hard." The despaired face Senku made at that was so heartbreaking that Tsukasa instinctively wrapped his arms around him. When he felt something wet on his shoulder he pulled him even closer. "Senku..."
The scientist pushed him away and quickly rubbed his eyes dry. "Shut up. I'll prepare you the most demonic diet plan so you better get strong fast!" He said making Tsukasa smile. "Anything you want." He replied running a hand through Senku's hair.
The days went on and they settled into a new routine. Tsukasa would drop off Senku to school in the mornings, do a little training, go to his matches or watch them if he wasn't scheduled for any and pick up Senku when his classes ended.
They'd walk home talking about their day. When they reached to the Ishigami's house or rarely, Tsukasa's house, they'd have dinner and Senku would talk to him about his new researches and experiments as Tsukasa would work out.
Senku would only tutor him on school work on weekends after they'd try out whatever experiments he had in mind. Thankfully, Byakuya didn't mind Tsukasa being around more than usual and staining few furnitures with blood. The man always made sure he felt welcomed and thanked him for taking care of his son.
//
Senku held something in his hands and quickly threw it to Tsukasa. The fighter snatched it and looked at the key in his hand. "It's so you can patch yourself up without having to wait for me." He explained. Tsukasa clutched the key. "Senku..." He smiled and added. "I'm still picking you up after school though." The white head chuckled. "You better."
It only took Tsukasa 3 days to abuse his use of the key. He barely went back to his house.
Senku's room was already his home. He had his own belongings mixed with Senku's in no time. The nightstand next to their bed had his protein supplements stacked next to Senku's small worn out notebook to write any ideas he had during the night.
Their clothes were hanged unorganized in the small wardrobe. The drawer next to the scientist's tools was filled with painkillers, creams and bandages for him. He even had a small sandbag Senku made for him in the corner of his, no, their room.
Byakuya told him that he was already a part of their family after he asked him if it was okay that he had a key to their house. It made him more than happy.
He remembered how the first few times he stayed over, Senku would grumble about having to clean up the clutter on the ground so he could sleep on a futon. Now, they didn't even mention it. His place was next to Senku on the cramped bed.
Byakuya offered to get him his own bed once but Tsukasa refused. He boldly said that he didn't want to be away from Senku. Byakuya gave him a huge smile and said he was glad Senku found a life long friend like him, although Tsukasa didn't really understand why, he seemed strangely happy.
This was the happiest he felt ever since Mirai was admitted. He had a soft smile on his face and focused on the homework Senku gave him before he heard him chuckling devilishly looking at his computer. "What are you up to now?" He asked from his place on the ground.
Senku turned to him with a huge smile. "I finally got an answer to my mails! And from a Nasa scientist!" Tsukasa felt excited for his friend. "Really?! Show me too." He walked over and looked at the screen. "Oh...It's in English." He said, dissapoined.
"Heh. I'm already studying it. I can't wait to try out his suggestions!" He said and immediately got to work as he explained to him what Dr.X suggested him. Tsukasa listened and hummed along. Seeing Senku so excited and happy made him feel warm. But there was part of him not really liking the situation. He buried those thoughts deep. He didn't want to ruin Senku's happiness with his useless comments.
When Senku was ready to test his small rocket, it was already night time but being so excited, he couldn't wait until the morning so they walked to the closest riverbank. Senku's hands were empty except for a notebook and a pen. Tsukasa, as usual, was carrying everything.
When it failed to take off properly, Senku smiled and wrote down his observations, not deterred at all. Tsukasa already knew how important failure was from being around the scientist so he silently watched to not interrupt Senku collecting pieces.
After a few more tries Senku started to yawn and Tsukasa hauled him back inside despite his protests. "Come on! At least let me mail my findings to Dr.X, it should be morning in America. He won't reply as fast if I wait until morning." He whined.
Tsukasa sighed. "Fine. But don't think of getting used to staying up late so it's more convenient for Dr.X." Senku grinned and rushed to type his thoughts as Tsukasa changed into comfortable clothes and laid on the bed. When Senku finally climbed over him to squeeze between him and the wall, Tsukasa was finally comfortable enough to fall asleep.
The next day he woke up earlier than usual because Senku was trying to slip away from his arms. He automatically tightened up. "It's 6 am." He stated after glancing the clock.
Senku glanced at him annoyed. "So what?" Tsukasa didn't reply and stared at him until Senku turned away with a blush on his face. "I wanna see if Dr.X replied." The brunet hummed and allowed Senku to slip out. "Fine. Not like you'd be able to go back to sleep with how excited you are." He said. Senku immediately rubbed the sleep away from his eyes and checked his computer. He brightened up seeing the response.
Tsukasa watched him typing away and taking notes. He too decided to have an early start and went ahead to take a shower.
When he got out Senku was still in the same spot, "You're still talking with him?" He asked. Senku seemed to absorbed in his chat with the scientist to hear his question. Tsukasa felt irritated enough to leave him alone and prepare breakfast.
When Senku finally got up, he was too busy reading a thesis Dr. Xeno wrote and kept reading it as Tsukasa took him to school.
The older knew how Senku got when he was interested in a something or even rarely, someone so he kept quiet. His excitement about his new mentor would pass in a few days. Or so he thought.
By the day 3 of Senku either ignoring him or talking about Xeno's achievements and thesis, he was fed up.
What bothered him wasn't that he thought Xeno was evil or anything, no he was happy Senku had someone to inquire about everything he thought. Just that it was the first time Senku was so deeply interested in someone other than him.
He hated how Senku seemed to enjoy talking to Xeno more than him and it was making him go crazy. He never felt that their friendship might not be as secure as he thought until now. Jealousy was eating his heart.
Was Senku getting bored of him? The thought made his head go blank. No. He wouldn't allow that to happen. He'd gain back Senku's attention one way or another.
Good thing he had a match today.