How Using My Oppai Skill to Enlarge My Sister’s Breasts Led to Building a Harem - Chapter 6

Chapter 6

Summer break ended, and the new school term began.

I’m a plain guy in my class, with no close friends. But when I entered the classroom, something felt off. Classmates were glancing at me, whispering secretly.

In that awkward atmosphere, a female classmate approached me.

“Hey, is it true you’re dating a model?”

Where did that rumor come from…? There’s no way a guy like me would—wait, Satsuki-chan mentioned something about being in a magazine…

“Uh, if you mean Sasahara Satsuki, yeah, we’re dating.”

“No way! You’re Satsuki-chan’s boyfriend!?”

Apparently, Satsuki-chan’s become famous. She’s blown up on SNS recently, seen as a trendy influencer among stylish girls.

“You’ve got a different vibe now… kinda cool?”

“Really? Thanks.”

It might be flattery, but being complimented felt nice. Then she whispered softly.

“You’re doing it with Satsuki-chan every day, right?”

“Huh!?”

How does she know that? As I reeled, she continued.

“Satsuki-chan said so.”

Satsuki-chan’s the type to casually share stuff like that on SNS. Being open’s fine, but this is a bit scary.

Too late, though—the rumor that I’m dating a model and having sex with her spread like wildfire.

※

“Aren’t you the talk of the school?”

“Huh?”

A few days later, the girl in front of me spoke up.

“Oh, Tsukihara-san. What rumor?”

She’s Tsukihara Yukino, the class’s unofficial “committee chair.” Not an official role, but her serious demeanor and glasses earn her the title. She’s one of the few who talks to a nobody like me.

“I heard you’re dating someone… and maybe engaging in improper relations?”

“Improper…? What’s that mean?”

“W-Well…! It means doing, uh, naughty things…”

“I get that, but calling a physical relationship ‘improper’ feels outdated. Don’t you think?”

It’s not incest, so it shouldn’t be impure.

“R-Really…?”

“Her parents approved our relationship. Is there a problem?”

“S-Sorry…”

Of course, my parents approved too. Ai, as her best friend, pushed hard for it, and Satsuki-chan’s charm won them over—they were thrilled.

Knowing it’s parent-approved, Yukino blushed and apologized. I wasn’t blaming her, though—she’s just a friend (maybe?) looking out for me.

“Thanks for worrying, Yukino-san. You were concerned about the rumors, right?”

“Y-Yeah… that’s it.”

“Thanks. You’re always looking out for everyone.”

“N-No way!”

Her face got redder, embarrassed now.

“That’s all! Gotta go!”

“If you hear more rumors, let me know?”

“Ugh… fine. Really, I’m sorry.”

Yukino agreed, looking guilty. She’s overly serious but genuinely kind, trusted by teachers and often tasked with class duties.

(She seemed kinda shaken…)

Her slumped shoulders as she walked away caught my eye.

※

Since our seats were close, Yukino and I started talking more.

“Kids these days are so advanced…”

“Are they?”

“I mean, your girlfriend, Sasahara-san…”

Serious Yukino had been researching modern relationships on SNS and short videos, slowly shifting her views.

“They say it’s rarer for girls not to have done it…”

“Maybe… I don’t know any girls around me who haven’t.”

“R-Really…?”

Yukino, who seemed inexperienced, listened intently.

“This is just my experience, so it might not apply, but…”

“Uh-huh.”

“I think women who are good at sex seem happier.”

“R-Really?”

Mizuki-san, Misa-san, Ai, and Satsuki-chan—all skilled in bed and super attractive. Guys love women like that, so being good at it’s a win.

“Strong women are proactive about sex.”

“I kinda get that…”

“And if you’re bad at it, it’s tough. You wanna please your partner, right?”

“Y-Yeah, true…”

I think Satsuki-chan said something similar in a video. Or am I just parroting her? Influencers’ words can feel like brainwashing.

After a few such talks, a few days later…

“Can I ask you for advice…?”

“Advice? Sure.”

Lately, Yukino seemed troubled. With a serious expression, she told me to follow her.

(I hope it’s something I can help with.)

She stopped in front of a room.

“The library?”

“Technically, the old library. It became a storage room after the library moved.”

Apparently, only people fetching old documents come here.

“Come in.”

Trusted by teachers, Yukino could borrow the key freely. She wanted to talk here. Once inside, she locked the door.

“So, what’s the advice?”

“W-Well… I’ve always felt out of place among the girls…”

“Really?”

“Yeah. I can’t keep up with their gossip, and I don’t have any real friends.”

I didn’t see her that way, but if she says so, it must be true.

“So I thought I need to catch up with the times…”

“That’s a great mindset.”

“So… could you teach me about sex…?”

“…Huh?”

Rubbing her thighs together shyly, she looked up at me. That expression, so unlike her usual self, was insanely alluring, making my heart race.

“Sorry, I had no one else to ask…”

“N-No, if I’m okay, I’ll help.”

The sweet, unfamiliar vibe made my head spin.

“W-What do I do first?”

“Let’s start with appeal. Can you take off your underwear?”

“Underwear… here?”

“You can’t have sex without taking it off.”

Blushing hard, Yukino reached under her skirt, slowly sliding down her pure white panties.

(Watching the serious class rep strip her panties is so thrilling.)

She hid them shyly.

“Oh, can I see the panties?”

“Huh!?”

“I need to check if they’re wet.”

If she’s wet easily, great. If not, she’ll need foreplay. I explained.

“I-I see… Okay… H-Here’s the part that… touched my, uh, place.”

“Not ‘place,’ pussy. You gotta say it clearly for sex.”

“Ugh… This is the part that touched my… p-pussy…”

“Good. Let me check. Sniff… Got it.”

Her panties weren’t wet with arousal, but a faint yellow stain and slight urine scent suggested nervousness.

“You don’t get wet easily, huh? Ever played with your pussy?”

“P-Played with… N-No, that’s embarrassing…”

Smart as she is, Yukino follows logic.

“It’s important. If you’re not wet, the cock won’t go in.”

“Auu… I-I’ve done it a few times… in the shower…”

Even the serious class rep masturbates. Interesting.

“Try it like you do then.”

“Here…?”

“Yup, so I can see.”

“Ugh…”

Understanding my logic, she spread her plump thighs, muchi♡, showing her panty-less skirt.

“How’s… this? Can you see?”

“Yeah, clear view. Beautiful pussy.”

It was gorgeous—hairless, smooth mound, puffy lips, her tight slit trembling, hikuhiku, from nerves.

“My… p-pussy… isn’t weird?”

“Nope, just like everyone else’s.”

“R-Really? Good.”

“Now, spread your pussy for me.”

Her closed lips hid her hole, so I leaned in close to see.

“Y-You do that…?”

“Yup. Satsuki-chan always spreads hers and says, ‘Go ahead.’”

“I see… Okay… Nn, so embarrassing…♡”

Shyly, she spread her pussy lips with her index and ring fingers, kupaa♡.

“Aah… you’re looking… haa…♡”

Turned on by being watched, her virgin hole dripped honey, torori. Her lewd display made my cock throb painfully against my pants.

“My cock’s getting tight, so I’ll strip too.”

Standing in front of her, I unbuckled my belt, kachakacha, and yanked down my pants.

“Hii!?”

I pulled too hard, and my erect cock grazed her nose, smacking my abs, bachin.

“S-Sorry, did it hit you?”

“N-No, I’m fine… This is a penis…”

“‘Penis’ is too formal. Call it a cock.”

“C-Cock… big cock…”

She stared at it, entranced.

“Big guys make women happy, right?”

“Satsuki-chan said that?”

“Yeah, she bragged about her boyfriend’s big c-cock.”

So my cock’s size is part of the rumors? That’s embarrassing…

“Getting it from a big one releases female hormones, makes you cuter, right?”

“Maybe, depends on the person… but good sex is exercise, so it’s healthy.”

I stripped my shirt, showing off my newly toned body.

“Wow… no wonder Sasahara-san brags.”

“Lots of sex builds muscles for sex, makes you healthy and attractive.”

“I want that…”

Yukino seemed to lose her aversion to sex, growing bolder.

“Let’s play with your pussy.”

“O-Okay… Nn, nn… Doing it while you watch… haa…♡”

Her slender fingers teased her pussy, kunikuni♡, distorting the hairless slit, revealing glimpses inside.

“That how you do it?”

“Y-Yeah… with the shower… like this… haa♡”

“Do it daily?”

“Auu… lately, maybe… daily. Sorry…”

She lied earlier but apologized—such a serious girl.

“Don’t you touch your clit? Most girls like it.”

“It’s… too sensitive… but, nn, ahn! Nhaa…♡”

Urged by me, she shifted from her hole to rolling her clit. The intense sensation made her jump, bikuri, letting out a sweet moan. She’d opened a new door.

“Move faster.”

“F-Faster… uu♡ Ah, ah, this… amazing♡”

She rubbed her clit furiously, her face turning lewd.

(Wow, Yukino-san’s face is so sexy…)

A girl lost in pleasure is so alluring. My cock swelled with boiling desire.

“Oh, my cock’s getting bigger…”

“Your appeal’s working. That’s how a guy’s cock grows.”

“I-I see… nn, nn, nnu♡”

Her clit play produced enough honey to drip, torori, pooling on the floor.

“Ready to start sex?”

“O-Okay… What do I do?”

No bed, just a table or chair.

“I’ll sit on the chair, you sit on me.”

I grabbed a chair and sat. Beckoning her to face away, her loosened pussy dripped clear liquid, tsuu, onto my cock’s tip.

“I’ll put the tip in. Sit slowly.”

“Got it… Nn… hot.”

My glans lined up with her entrance. She lowered her hips.

“Ah, ouch! This is my…”

“Virgin membrane. It’s fine, it’ll feel good soon.”

“Really…? Ow, it hurts…”

To ease her pain, I grabbed her soft breasts, funyu♡.

“Nn! Rubbing my chest? Haa♡ No…”

I kneaded her perfectly balanced, innocent breasts gently. My palm sensors detected their firmness—too much force would be bad. I stroked from base to nipple, shu… shu, along the nerves.

“Fuaa♡ That’s intense… my nipples are tightening…♡”

“Gather the pleasure in your nipples… here!”

I milked the pleasure to her nipples, pinching them, kyu♡.

“Ah, auu! Ah… kya!”

Her legs gave out from the nipple pleasure, her hips dropping, my cock piercing her hymen and hitting her cervix, sudon.

“Zuu… chun♡”

“Ah… aaah…”

Stunned by the pain and impact, she couldn’t process it. But the pain would hit soon. To spare her, I pinched her hypersensitive nipples hard, gyuu♡.

“Yiii…! Aaah♡ Ohh♡ What’s this… what’s happening…♡”

Pain and pleasure surged simultaneously. I’d sent pleasure through her breast nerves just as the pain hit, confusing her brain.

Skewered on my cock, she twitched, biku biku♪, nearly falling off.

“Whoa, careful!”

“Ah… thanks…♡”

I lifted her by the knees to steady her. As her mind cleared, I spoke.

“See? My cock’s in your pussy.”

“Huh… ah…”

Her legs spread in an M-shape, my thick cock visibly inside her once-virgin pussy. The sight tightened her pussy, kyu♡.

“Kuh… Yukino-san, your pussy’s tight!”

“D-Don’t say that… showing me something so lewd…♡”

“Loosen up, or I’ll come!”

I’d inserted into her tight, virgin pussy, but my fingerwork sent too much pleasure, making her pussy writhe. The sensation was like full-on sex, and cum surged from my balls.

“Come…? What… ah, hot!♡♡♡”

“Auu♡ Something’s shooting… pulsing…!”

My cum erupted into her pussy. Stunned, she took my repeated shots, byu♡.

“Aku…! Can’t stop… this pussy’s too good!”

I came inside her unprotected pussy, unable to stop from the overwhelming pleasure.

Pachun♡ Pachun♡ Pachun♡ Pachun♡

“This is sperm…? Why… ah, ah♡ Coming while you pound my pussy♡”

“Sorry, Yukino-san! I can’t hold back!”

Unable to resist her tempting pussy, I thrust hard, zokuzoku♡, toward her womb.

“Hiii♡ N-No… that’s where babies come from!”

“But it feels good, right? Here?”

I targeted her womb’s sweet spot, hitting the best place hard.

“Yiii… there, no…♡”

“Here’s good, right? Take it!”

Dochun♡ Dochun♡ Dochun♡

Her portio was perfectly positioned for my cock, the glans hitting with every thrust.

“Our bodies are super compatible!”

“Oh, oh, ohh♡ No… but…♡”

“Feel it all!”

“I’ll… make a baby…♡”

She didn’t know an emergency pill could prevent pregnancy. Thinking she’d conceive, she seemed half-conscious.

“It’s okay, Yukino-san!”

“Ah♡ Ah♡ Ah♡ …Okay? You’ll take responsibility…?”

“No, I mean…”

“I’ll have a healthy baby…♡ Nnuu~~~♡♡♡”

Something snapped in her, and she ground her pussy, guriguri, against my cock. Finding her adorable, I pounded with full force.

Dochu! Dochu♡ Dochu♡ Dochu♡ Dochu♡

“Ohhh♡ Amazing, amazing…♡ Yiii!”

Her perfect pussy took my brutal thrusts, making her writhe insanely. My balls churned, producing cum, urging release.

“Kuo, coming again!!”

“Nghii♡ My head’s breaking! Ahhh~~~♡♡♡”

“Ohh♡ Hoo… feels good… something’s bursting in my head…♡”

Dopamine rush, maybe? She was high on extreme pleasure. I poured fresh cum, dobadoba, into her pussy, watching her drool in bliss.

(This face is cute too…)

Compared to her usual sharp class rep look, she was way more charming now.

I hugged her trembling body from behind, savoring the sweet afterglow of ejaculation.

…

“So… I don’t have to have a baby?”

“Right. Take this emergency pill to avoid pregnancy.”

It should prevent conception if taken soon, though it’s not 100%…

“Okay… I see.”

She looked relieved but oddly disappointed.

(No way she wanted a kid… right?)

We’d only talked a few times since being classmates—not a romantic thing.

“You’re the only one who really talked to me…”

“Huh?”

About feeling out of place, probably.

“And then, after summer break, you got so hot…”

“…?”

“The girls in class were after you…”

Probably ‘cause I’m the “model’s boyfriend.” Real feelings wouldn’t shift just from a glow-up. Not like her.

“I panicked… so I did this…”

“Asked me to teach you sex?”

“You said you like girls who are good at sex…”

“Well, yeah…”

I do like Yukino in that sense. She’s cute, makes my heart race, and the sex was amazing.

But… I have Ai. And Satsuki-chan. I can’t just date her.

“So I thought, maybe if I had a baby, you’d take responsibility…”

“What!?”

“Just for a second… I thought that.”

A joke… right?

“Here’s the pill… Gulp…!”

Her throat moved. She took it, right?

“Yukino-san, open your mouth.”

“Caught me?”

She showed the pill on her tongue.

“Please, you’ll really get pregnant…”

“Fine… but make me your girlfriend. Just at school.”

“Just at school…?”

That’s a sex friend. Feels familiar.

“If you don’t, I won’t take it.”

“Okay! Be my girlfriend!”

“Good. Call me by my name from now on.”

“Got it, Yukino-chan?”

“Yukino!”

“Alright, Yukino.”

“Yay♪”

And so, I got a second girlfriend.

(Just a school girlfriend… so it’s light cheating, right?)

A bit scared by Yukino’s boldness, I spent the rest of the day flirting with my new girlfriend until school ended.