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It was easy to see Senku wasn't intimidated by Tsukasa in any manner, regardless of the latter's strength and past actions. The frail scientist often teasingly poked at him when he said something sappy, or twirled the brunet's hair around his fingers while he was preoccupied with some sort of research. Once, Senku even convinced Tsukasa to lift him onto his shoulders so he was able to reach something on a shelf much taller than both of them individually—when Ryusui volunteered to do the same thing, Senku flailed about and nearly kicked him (not that it would've done much).
Unfortunately, the way Senku flinched when Tsukasa's hand so much as brushed his shoulder told a different story.
Tsukasa understood, of course. He couldn't expect Senku's mind nor his body to forget what he'd done, regardless of how long ago. He understood very well—oftentimes, when the two made contact, Tsukasa once again felt a frail bone shatter under his hand.