While waiting for the manga to progress further, I thought it'd be a good idea to do a special thing for the other boys! So this will be a compilation of moments in the story where some romance happen. Okay, maybe not ALL of them in case this gets too long.
Just so you know, in this alternative timeline, there's no already-existent crush on Senku.
We're going chronologically! Gen's first!

[In the 'Bloody and Interested' chapter]:

"It's a bag full of resin's juice." Senku said after taking out said bag which protected Gen from any fatal damage. "It protected him!"
You let out a long sigh of relief.
"He's got them all over his body!" Suika exclaimed.
"These are crazy!" Chrome looked closely at one of them. "From the moment he entered the village, he was prepared for the worst! And he went all out protecting himself with these!"

As you listened, you stood up and walked over to Gen. Looking at him with furrowed eyebrows, you felt a new found respect towards him.
That stunt he did with the flowers made you realize that he wasn't just a scary dude. It even led to him getting brutally attacked, and you felt so bad about it.

...

"The bandage must be replaced right before going to sleep, and as soon as you wake up." Senku reminded you as he walked out of the shed to sleep outside.
You nodded and wished him goodnight before turning back towards Gen, sitting beside him, your expression more grave.

"It's our fault you were put in this situation." you bowed. "On behalf of the Kingdom of Science, I sincerely apologize." you sat back up, now scratching your cheek, looking away from him. "And... I'm sorry I acted weird around you, and called you a fuckin' idiot. It's just-" you sighed. "It was stupid."
Gen would've chuckled if it wouldn't hurt. Instead he smiled and shook his head.
"It was fun." he said with a weak voice.
"You know what? From your point of view, I bet it was!" you smiled. "But please, don't speak so much. Now."
You then began to replace his bandages. The only thing that could be heard was the chirping of birds outside.

...

The sun hasn't even risen yet. Everything was dark, everything was quiet. Gen clumsily stood up and tried to replace his bandage by himself, as quickly and quietly as possible, though his wounds still hurt like hell, he went through it.
Then he froze when he heard you turn in your sleep, moaning lightly. He slowly looked at you, then down and his bandage. A moment passed.
"Oopsie." he said to himself as he let go of it and started wrapping it around himself in the most messy way possible.
After making sure he looked like he royally fucked up, he went to you and gently shook your shoulder.

"(Y/n)-chan~, (Y/n)-chan~." he whispered.
You let out another moan before opening your eyes. Rubbing them, you looked up at him, your sleepy look making him smile, it took you a second to be fully awake.
"Gen?" you sat up, then frowned when you noticed his bandages.
"I tried doing it by myself but..." he shrugged, smiling. "Could you help me~?"
"...You're leaving?"
"Why yes. I have to hurry back before Tsukasa-chan gets suspicious." he noticed the look on your face and leaned a bit closer. "Aw~ are you worried about me?"
"Wha-?!" you backed off and looked away. "I-uh..."
Gen giggled. He already had his answer.
"Hurry~." he crossed his legs and presented his arm, where the end of one of the bandages was. "I have a long way ahead of me."
"Right."

You took the bandages off and started to replace the healing paste. You went as quickly as possible, you were silent in your focus.
"You know." Gen eventually broke the silence. "Insecurity doesn't suit you."
You froze, caught off guard. You opened your mouth but couldn't think of anything to say. You were behind him, but Gen could very well picture your expression. He chuckled.
"It's a shame really."
It took you a moment to process his words, you almost expected him to maybe develop on that last sentence, but he stayed quiet. You continued with your task, also silent, feeling your cheeks getting warmer.

[In the 'Guess who's back!' chapter]:

"Yeah but can that thing reeeeeally start a fire? Like for realsies?"
"Yeah, it's called a burning glass!" Senku explained. "Fires have even been started by convex lenses created from suction cups and air-." he suddenly stopped.

You both turned around to see Asagiri Gen, standing on top of one of the poles holding the bridge. You loudly gasped.
"Long time no see~!" he waved.
"You weasel son of a bitch! How did you get up here?!"
"Oh (Y/n)-chan~! I just got back after such a long journey and you welcome me with insults?" he wore a hurtful expression, though he still genuinely hated the weasel thing. "And after this genuine and romantic moment we shared right before I left!"
"You-Fuck off!" you blushed. "It's been a rough day okay?!"
"Romantic moment?" Kohaku curiously looked at you.
"Whatever! He's a weasel!" you exclaimed, only getting redder.
Seeing such a reaction made him giggled as he hopped down the pole.

...

"Ma boy!!!" you ran over to Chrome, and so did the others.
You ruffled his hair, but then had to step back from the group to calm yourself from this elevator of emotion you've just experienced.
"I've never seen you this happy before."
You looked over at Gen.
"Yeah." you sighed, smiling. "The rough day's over!" you paused. "Hey, thanks for your help. You really saved our asses on that one."
"It's nothing~. Like I said, I had cola riding on this." he leaned closer. "Say, I'm curious, did you miss me?"

"W-what?" you blinked twice.
"I 'saved your asses' as you so elegantly put it." he smiled innocently, though you could see the mischievousness behind it. "I get one bit of honesty from you. So did you miss me?" he tilted his head. "You know I will be able to tell if you're lying or not."
Your eyes darted around, panicking on the inside for a moment. Then you sighed.
"I...I am glad your wound didn't get worse." you said without looking directly at him.
"Mmh~." he narrowed his eyes.
'Stop staring at me!!!' you frowned.
"You know what?" he leaned back. "I'll take it!"
You sighed, feeling relieve. Though your heart needed a bit more time to calm down.

[In the 'The first attack' chapter]:

"Who's the strongest?" Gen asked after picking up a rock.
"Magma." you quickly replied, understanding what he was planning.
The two of you rush to him.

"Alright Magma-chan!" he handed him the rock. "Throw this at the enemy with everything you got!"
"Huh?!" he frowned. "There's no way I'd hit them with this!"
"It's fine if you miss."
"Yeah we don't care!" you nodded.
"And why should I listen to you two?!"
Gen's smile became more sinister.
"You'd love to steal the title of chief from Senku-chan wouldn't you? You don't wanna lose the whole village now do you? We've got to cooperate for now."
"Yeah!" you added. "And your actions and cooperation in our projects would give you massive popularity points. What do you think of that?!"
Magma took the rock and walked away to get into position. You let out a sigh of relief.

"(Y/n)-chan~!" Gen cheerfully said. "I'm so proud of you~!" he squished your cheeks.
"What are you-Would you stop?!" you slapped his hands away, blushing, and quickly walked back to Senku, hoping Gen didn't notice the short seconds were you looked genuinely embarrassed.
Oh but Gen DID notice! And he was gonna cherish that sight!

...

The group on the other side of the bridge was suddenly in a panic. While Gen ran to join them, you stepped forward, putting on an enraged face.
"Gen you bastard! You were with them this whole time?!"
"...I'm sorry."
He turned to look at you, but his expression was softer than you expected.
"But, would you believe me if I said this never changed the way I feel about you?"
Your eyes widen and you quickly turned back while he kept running away. You covered your red cheeks.
'This man! What was that about?!?!"
Normally, you'd smile at such a cheesy line. But right now, it just made your heart skip a beat.
"What the fuck...?" you mumbled under your breath.

[In the 'Favorite day on the year' chapter]:

"Gen! Holy shit look!" you rushed towards him, excited to show your present. "Senku made me a trumpet! It's so sick~!"
Though Gen liked seeing the stars in your eyes as you looked at your new instruments, he didn't like the fact that Senku was the one that got that cute reaction out of you.
"That's great (Y/n)-chan!" he placed his fingertip on his chin. "Man~ I feel bad, I don't have a present. Oh wait, actually I do!"
"What?" your eyes widen.

He held up his hands and stayed motionless for a moment. Then suddenly he turned them around and a flower appeared in one of them.
"Ta-da~!"
"Aw~!" your eyebrows furrowed as you took it. "Thank you Gen!"
"You're very welcome! Normally I'd charge my dates for a magic trick, but I'll make an exception for you."
You chuckled, you couldn't tell if he was lying or not.
"Oh man! Now that's two presents I can't give anything in return!"
"True." he leaned closer, his smile grew larger. "I can let you off the hook for a kiss though~."
"Huh?" your eyes widen and you blushed.
He giggled.
"I won't force you if you don't want to." he waved his hand. "But to keep the Christmas-transaction business going, I'll need something in return at one point or another."
"What are you talking about?!"
The two of you laughed.

[In the 'Winter goodness' chapter]:

"In other words: get to work, huh?" Gen mumbled as he sat beside you.
"What's your deal with working Gen? You know you can't just sit around." you said.
"Why not~?" he leaned over and rested on your side. "Why can't I just get my little harem and enjoy life as others do the work for me?"
"Get off me! You're heavy!"
"Your blanket is so warm though~!" he snuggled closer. "How about we share?"
"Get your own blanket dude!"
"Come on~." he looked up at you. "I'll take it as my Christmas present! Now it's either that or the kiss!"
You narrowed your eyes, blushing.
"...Hop right in."
"Yay~!" he rolled over to sit close to you so that the blanket covered the two of you.

Ruri chuckled.
"You two have gotten a lot closer huh?"
"You could say that yes!" Gen cheerfully responded.
You just didn't respond and pretended to be focused on your work.

...

"Wait, what's Halloween?"
"Oh it's THE best Holiday of the year!" Gen exclaimed.
"Wha-You did not just say that! How dare you?!" you looked at him, smiling. "After everything I just said?!"
"I can't control how I feel!"
"I cannot believe you!"
"I can't believe you prefer Christmas over Halloween!"

You exaggeratedly gasped and took the blanket for yourself.
"I don't think this is working out anymore."
"Wha-?" Gen scoffed, breaking character for a second before following you in that direction. "Yeah me neither! My mother was right about you!"
You burst out laughing. Gen laughed too, but he really felt as if you both passed a point where you can now joke about...whatever's going on between you two.

[In the 'One-sided honesty' chapter]:

"I have a question!" Gen suddenly stepped closer to the mic. "How exactly did you convince Tsukasa-chan that you were fresh out of the stone?"
"Uh...I-uh..." you became as red as a tomato, now double embarrassed because everyone was listening. "H-he lend me his coat."

"(Y/n)-chan are you cheating on me?!"
You couldn't help but laugh, actually feeling glad he was joking about it and not taking this so seriously.
"What? Jealous?!"
"Of course I am!" he quickly replied.
Oh-ho, he sounded serious. Your heart skipped a beat.
"(Y/n)?!" Kaseki stepped in, furious like he was your fucking dad. "What did you do-?!"
"Okay gotta go bye!" you quickly put the cellphone back underground.

[In the 'You win some you lose some' chapter]:

"And do you have any basis on which I should trust something as hard to believe as that?" Tsukasa eventually asked.
"...only my words." Senku said. "Science does not lie, is that not enough?"
"No. It's plenty...more than enough."
You smiled.

"Now that everything is cleared up and we're all friends." Gen said, walking up to you and hugged you from behind, resting his chin on your shoulder and looking up at Tsukasa. "I'm sorry Tsukasa-chan, but (Y/n) is mine~."
"Huh?!" you were as shocked as Tsukasa.
"That's...unexpected." he said.
"Well you know what they say: the heart has reasons that reason cannot know!" he proudly said.
"G-Gen please." you managed to say.
"Aw~ embarrassed?" Gen smirked, looking at you.
"No! You're just way to fuckin close!"
"Alright~ alright~." he stepped back, his hands held up. "Not in front of others, I get it."
"Stop saying this kind of stuff!"

...

When Gen pieced together that Hyouga was bad news, a chill ran down his spine when thinking about how he disliked you and whatever he'd do once Tsukasa isnout of the picture.
When he ran alongside the others to get to you, he didn't realize that his adrenaline rush made him go faster than usual.
As soon as he saw you in one piece, he felt overcome with relief. He immediately hugged you tightly. It took you a moment to process what was happening, but you hugged back right after.

"You're okay. You're okay." Gen quietly said.
You didn't know if he was comforting you, or himself.
"Tsukasa..." you said with a trembling voice. "And Senku."
"They'll be fine, you know them." he smiled and let go of you, still holding on to your cheeks and wiping your tears away with his thumb.
You smiled back. He kissed you on the cheek. You gasped and blushed.

[In the 'Confession' chapter]:

"So, the night after we made the guitar, he asked me to speak privately..."

"(Y/n)-chan."
"Uh...yeah?"
"We need to address what's happening between us."
"What?"
"You like me."
"Huh?!"
"And I like you."
"What?!"
"Don't play dumb~." he waved his hand. "It's not like I try to hide it. And you know I'm good at throwing you off."
"Yeah, 'cause right now I'm thrown off! You're giving me too much to process!"

"Okay, let's make it simpler." he shifted in his seat to face you fully "(Y/n)-chan, I like you. Do you feel the same way?"
A moment passed where you just stare at each-other. Your mind was a mess right now, you almost felt dizzy.
"I helped you tons of times," he held up his index. "I can get one bit of honesty from you, now can't I?"
"I...I-" you sighed.
'Why can't I fuckin speak?!?!'
You weren't normally like this, but with a guy like Gen, who was WAY too open and flirty about this kind of thing, you felt like you couldn't keep up. So the words just get stuck in your throat.

"Okay, let's say..." he paused. "I stop getting all flirty with you, would you be bothered?"
You thought for a moment, your mind went a bit blank when the first thing that came to mind was 'Yeah!'. You took a deep breath.
"Okay." you said.
"What?"
"You and me. Let's go."
Gen blinked twice and burst out laughing.
"I never...Who would've thought that you'd get all tsundere on me?!"
"That's just because you...! Like...! Whatever!"
"Aw~!!! You're so cute!" he hugged you.

"...I am glad though." you hugged back. "I wasn't sure if you were serious about this or not."
"Of course I am!" he stepped back and looked at you, still holding on to your hands. "Like I said, I'm not trying to hide it!"
"But it's *because* you're so open about it that I wasn't sure if you were joking or not!"
He smiled.
"If I kissed you, would it convince you that I'm serious?"
Your eyes widen. And your heart went bunkers when he closed his and began to lean closer. You closed yours and waited.

He booped your nose.
"Got'cha~."
"Oh you ass!" you shook away his hands, your face as red as a tomato, and turned away. "Already deceiving in the first minutes of our relationship!"
Then, he placed his hand on your cheek and gently turned your head to gently place a kiss on your lips.
"Double got'cha." he smiled. "Told you I was serious." a moment passed. "...(Y/n)?"

He waved a hand in front of your face, you were frozen into place. Gen actually started to get concerned when he was started by your sudden hug.
"Oh you fuckin weasel!" you said, pressing your face on his shoulder.
He laughed and hugged you back.

So, Gen as a boyfriend:

Lots of kisses and flirt, Gen's very touchy-feely, when Ryusui steps in he immediately establishes that (Y/n)'s his girlfriend.
And she gets used to his shenanigans over time, even flirts back sometimes if she thinks of a good line. She blushes less and even sometimes ignores him on purpose for the sake of joking.
In the treasure island arc, whenever Mozu's not looking, Gen steals a kiss from her cause he got jealous as fuck and also he's scared for her (cause she clearly isn't!)

And that's kinda how it would go!