Work Text:
When people first met Senku, they were either charmed or completely put off and then charmed later. Despite his absolutely horrendous people skills, he had a knack for gathering others around him, eager to help and support his goals.
Regardless of a person’s first impression of him, it didn’t take long before they became aware of some of his more terrible habits. How he could speak with the flattest, most dismissive tone that made even the most innocent suggestion feel like an insult. The way he skipped meals or pulled all-nighters when engrossed in a project, becoming snappy and irritable the way an overtired toddler did. How often he shoved his pinky in his ear mid-conversation, completely unfazed by how gross or distracting it was.
But the bad habit that rubbed Gen the worst way was how his fingers always somehow ended up in his mouth to be gnawed on when otherwise unoccupied.
It was a fairly common habit, nail biting, and Gen doubted Senku even realized he was doing it. If Gen wanted to arm chair diagnose, he’d call it stimming- how Senku’s thumb systemically stroked over his finger tips, zeroing in on harder bits of cuticle or sharp nail edges and bringing them to his mouth to chew on.
Senku’s nails were short- jagged in some spots from being bitten off and framed by roughened bits of cuticle. His fingertips were scarred and calloused from years of soldering burns and working with small tools, dirt and grit gathered under the few nails allowed to grow long enough to have space and staining cuticles overgrown where teeth couldn’t nibble them away. They were working hands, tools of science, strong and sure.
As someone with a standing appointment every three weeks for a mani pedi before it all went to shit, the sight of them made Gen’s skin itch.
Sure, Gen’s own hands had seen better days, living rough and hours of manual labor taking their toll on the delicate digits, but he did his best to maintain them as much as possible. A small piece of bamboo shaped to push back his cuticles, some pumice pilfered from Chrome’s hut to keep his nails shaped and filed. He would do unspeakable things for some hand lotion, but one made do with what one had.
Gen wouldn’t have done anything about it if not for the most recent time he had caught Senku chewing on himself. The two were in the lab, Gen working on his next lesson plan for teaching the villagers to read and write as Senku looked over blueprints and schematics, left index finger caught between worrying teeth. Out of the corner of his eye, Gen saw how the scientist suddenly winced, pulling his finger out of his mouth and staring at it as if he wasn’t sure how it got there, nail broken below the quick and jutting out from where his teeth had bent it up.
Senku’s nose had crinkled as his lip curled up, thumb stroking over the jagged nail piece and cringing slightly as it pulled, bringing it back to his mouth to finish the job.
Gen didn’t realize he was moving until his hand curled around Senku’s wrist, stopping him in his tracks.
“You’re going to give yourself an infection,” he said, tugging the scientist’s hand towards him to get a better look. His cuticles were rough and irritated, and the broken flap was already beginning to pull at the top layers of the still-attached nail, leaving a ragged edge from his fiddling. “Do you have a knife? A clean one?”
“Gonna amputate?” Senku smirked, reaching over and handing Gen a small pocket knife.
Rolling his eyes, Gen inspected the blade closely and decided it was clean enough for his purposes. “Keep up with the nail biting and I might have to.” His grip shifted, fingers curled under Senku’s to hold his hand steady as he slid the blade across the break, catching the edge and slicing cleanly to remove it. The place where it tore was still rough, but the remainder was smooth, though Gen was sure Senku would change that soon enough the next time his hands were idle.
While he was at it, Gen inspected his other fingers, shaking his head slowly at the state of them. Senku tried to pull away, stopping when Gen’s grip tightened, thumb pressed to the back of his hand to hold him in place.
“For someone who uses their hands as much as you, you don’t take very good care of them,” he hummed, finally dropping his hand and catching its twin for another inspection.
It took Senku a beat to reply and Gen could’ve sworn he’d heard a sharp intake of breath, but when he looked up the other man’s expression was normal, if not a bit annoyed. “I take care of them fine. Wash them multiple times a day, clean any cuts or burns… Hands don’t have to be pretty to be useful.”
Gen rolled his eyes at that, thumb running over a particularly rough area of cuticle on Senku’s pinky before releasing his hold. “Something tells me a staph infection wouldn’t be useful, either.” He arched a brow at Senku’s annoyed huff, the other man already rubbing his thumb over the freshly cut nail. “While I would love to have you visit Gen’s Nail and Beauty Salon , I know that your work would undo any progress I made in fixing those talons of yours,” he wiggled his fingers at Senku’s hands in emphasis. “Biting your nails is an errible-tay habit to have, Senku-chan, you really should try to stop.”
“I’ll keep that in mind, Mentalist,” Senku snorted, Gen able to hear the “yeah I’ll just stop a compulsive behavior I’ve had for years because you suggested it” loud and clear through the roll of the other man’s eyes.
And so Gen let it go. For the moment, at least.
But he kept watching.
Senku’s brain moved quickly, and that extra energy needed an outlet. When his brain was chewing through data and his hands were idle, his mouth chewed keratin and cuticles. It was automatic, harmless back in the days of easily accessible nail clippers and dermatologists. The broken nail hadn’t even bled, but in the Stone World where the only antibiotic was a sulfa drug that most certainly was not going to keep MRSA at bay the habit was one that Gen knew he needed to get Senku to break.
He knew how stubborn Senku could be, though, and knew telling him to stop was just going to make it worse. It would introduce a subconscious desire to defy the order and most likely result in him biting his nails more often than before.
Gross tasting nail polish was out, as he’d need to ask Senku to make it and the scientist would just wave it off as unnecessary. Hot sauce could work, but Gen couldn’t help but cringe when imagining the sting in Senku’s irritated cuticles. Keeping them short didn’t seem to help either, he just kept nibbling on whatever he could reach.
No, this would require a different approach.
The next time Gen caught him examining some chemical equation, thumb caught between his teeth in thought, he simply walked over and took his hand.
Senku blinked, distracted, fingers curling loosely into Gen’s automatically.
“What are you doing?”
“Helping,” Gen replied, the look on his face daring Senku to argue.
Crimson eyes regarded him for a long moment and Gen felt the way the fingers linked with his twitched briefly, but Senku eventually turned back to what he was doing without letting go.
And when he didn’t pull away, neither did Gen.
It became somewhat of a game after that. Any time Gen caught him biting- during planning sessions, mid-experiment, mapping the stars- he would sidle up and take his hand. He didn’t make a show of it, never scolded or maligned, just interlaced their fingers and stroked his thumb over the back of Senku’s knuckles until the tension eased.
A few times, Senku had grumbled about needing his hand back to measure something or jot down a note, and on one occasion he had switched to nibbling the other hand requiring Gen to hold both of them at the same time much to the scientist’s chagrin, but he eventually stopped objecting.
What was more surprising was how after a while Senku began to preempt it.
The first time it happened was a quiet afternoon in the lab, Senku hunched over his roadmap for some impressive device and lost in thought. His finger twitched towards his mouth before pausing halfway, switching directions and reaching across the table where Gen was sitting without looking up.
Gen took his hand with a soft smile and a quirked brow, lacing their fingers together as if it was the most natural thing in the world. He didn’t say anything, didn’t tease.
Just stroked his thumb over the back of Senku’s and returned to his own task.
After that, Senku stopped chewing on his nails - at least when Gen was around. The habit didn’t vanish overnight, lingering in muscle memory as his fingers twitched towards his mouth in moments of deep focus. Something always stopped the motion, though, hand reaching out for Gen’s seemingly without conscious thought.
Sometimes it was casual, a hand stretched across a work table as his eyes scanned schematics. Other times it was subtle, fingers brushing against Gen’s under a table until the mentalist looked up and took them in his own. Every so often, he would even volunteer both hands to the mentalist as he tried to talk through his latest mental roadblock, Gen humming along and offering opinions as he took the opportunity to tend to them - pushing back cuticles and smoothing the roughened edges with a gentle precision.
Sometimes Gen reached for him first, able to read the tension in Senku’s jaw and the furrow in his brow before his fingers even began to twitch, and Senku never pulled away.
Neither made a spectacle of it. Gen never scolded or teased, just curled his fingers around Senku’s and anchored him there until the urge passed. When he needed both hands again, Gen let go without a fuss.
It became habit. Gen never asked Senku to stop biting his nails again, he didn’t need to.
He’d simply offered a better habit in its place.