(A/N: This Spin-Off chapter will have a bit of fan service for Adult Jack.)
...
-No One's P.O.V-
It was a normal day for everyone and we see Kazuma heading inside Wiz's shop.
Kazuma: Good morning, Wiz!
Wiz: Ah, Kazuma-san~! What can I get you for?
Kazuma: Oh, I'm just gonna roam around in your shop for a bit.
Wiz: Alright, take your time.
Kazuma then looks at the shelves around Wiz's shop to see if there is anything that is valuable to him in a quest until his eyes landed on a potion.
Kazuma: Hey Wiz, what's up with this potion with mixed-colored liquid on it?
Kazuma curiously asked.
Wiz: Oh, that's the Potion of Transformation.
Kazuma: Potion of Transformation?
Wiz: Yes, I heard that this potion that if you drink it, your current age will turn you into an adult, probably around either early or mid-20s.
Wiz explains as Kazuma thinks to himself.
Kazuma: (A potion that can transform you into an adult huh? Wait, what if I can put it onto Megumin? No, probably (Y/N). Yuuki? Or perhaps Jack? Hmm...I think Jack might have to do.)
Kazuma thought to himself until he spoke up.
Kazuma: How long will this potion last?
Wiz: Well, if you drink it today, the potion's effects might affect by tomorrow. However, it will only last for about 24 hours once it takes effect by tomorrow.
Kazuma: I'll buy it!
Eventually, Kazuma paid the Potion of Transformation for about 1,000 Eris.
-Time Skip to Evening-
So right now, Kazuma decided to plan everything to pour the potion.
Kazuma: (Alright, dinner will be ready soon. Jack might have to do the trick since she's a kid. Hmm...since Jack always sits next to (Y/N). I might have to pour a fresh cup of water before I put the potion in her cup.)
Kazuma thought to himself as he sneakily comes into the kitchen and sees Aqua cooking food until she notices him.
Aqua: Oh Kazuma, dinner is about to be ready soon. I'll call you out once it's finished, okay?
Kazuma: I know, I'm just checking the kitchen for a bit.
Kazuma excuses as Aqua turns back to cook the food. He then went up to Jack's cup as he used the skill Create Water to pour inside Jack's cup. After that, he poured the whole Potion of Transformation inside.
Kazuma: (There we go...)
He nods in accomplishment before placing Jack's cup back on the table and left the room.
Kazuma: Now then, we wait.
-Mini Time Skip-
Within a few minutes later, Aqua called out to everyone that dinner was ready and all of you began to chow down the food while Kazuma was intensively watching Jack eating her food until she grabbed the cup next to her and drank it.
Kazuma: (Okay, good, good...)
He thought to himself with a nod until Rin spoke up.
Rin: Hey Kazuma, why are you staring at Jack?
Kazuma: O-Oh, sorry...!
Megumin: You're not planning anything, are you?
Megumin asked skeptically.
Kazuma: No! Why would assume that I'm planning anything? I just...dozed off, that's all.
Kazuma lied with his hand in front of him as a defense.
Aqua: Okay then, but you need to eat your feed because it's gonna get cold.
Aqua tells him as Kazuma eventually ate his food. That's when Jack spoke up to you.
Jack: Hey mom, we don't feel well...
(Y/N): Huh? What's wrong?
Jack: We drank our cup and...our head hurts...
Jack says while holding her head in pain. This concerns you until you spoke up.
(Y/N): Hey, do you wanna lie down?
Jack: Y-Yeah...
You and Jack got off your chairs as you helped Jack to go to your room to lie down in the bed.
Darkness: That's strange. Jack only just drank her cup. Aqua, did you do something in her drink?
Aqua: No, not really. I was just cooking dinner this whole time.
Rin: Why don't we check her cup out?
Megumin then grabs Jack's cup and inspects it until she sees the mixed-colored liquid from the potion that was left inside the cup.
Megumin: Hey, I found something weird inside Jack's up.
Kazuma: What? What is it?
Megumin: There's some...weird mixed colors inside of her cup.
Aqua: What? Let me take a look.
Megumin hands the cup as Aqua looks inside Jack's cup before taking the mixed color liquid using her index finger.
Aqua: Huh, I've never seen anything like this strange liquid before.
Darkness: Are you sure you didn't put anything like that inside of Jack's cup?
Aqua: No I didn't! Why would I put this weird liquid inside while I was cooking?!
Aqua exclaims.
Kazuma: Can you taste it?
Kazuma curiously asks until Rin spoke up.
Rin: Wait, don't taste it. Jack had a headache with it once she drank her cup, let's not taste it.
Rin says as Aqua puts down Jack's cup before washing it off. That's when you entered the room as Megumin spoke up.
Megumin: How is she holding up?
(Y/N): She's fine. Jack still has a headache so I decided to leave her on the bed and it might go away soon enough.
You said as you eventually joined the group to eat dinner without Jack.)
Kazuma: (Alright, the potion is affecting Jack until the next day. Let's see what she looks like if she was an adult.)
Kazuma thought to himself.
-The Next Day-
Morning came by and you were sleeping peacefully in your bed until you hear a somewhat familiar voice speaking to you while gently shaking you.
???: Mom...Mom...!
It sounded like Jack, but a lot different. You turn over to your left and opened your eyes to see Jack's hair, but a lot more longer. However, your vision was a little blurry because of the sleep.
(Y/N): Morning, Jack...
You said in a sleepy face as your rubbed your eyes until your vision became clear. In front of you was Jack, but...it was an adult woman who looked exactly like Jack.
(A/N: Oh dear me...I really felt a little horny by looking at this image. Sheesh!)
Both you and the adult female looked at each other for a moment until your face started to red as you let out a scream.
(Y/N): AHHHH!! W-W-W-WHO ARE YOU!?!?
You asked while your back was pressed against the wall.
Jack: Mom, calm down! It's us! It's Jack!
She exclaims. That's when Rin woke up from her sleep and sees this.
Rin: HEY!! WHAT THE HECK ARE YOU DOING TO MY LITTLE BROTHER!?!?
Rin then summons a bunch of arrow bullets and fires directly at Jack, but Jack grabs you before dodging Rin's arrow bullets, which hits directly at the wall.
(Y/N): W-What...?!
Rin: Get away from him, you slime!
(Y/N): R-Rin, wait...!!
You said with your arm held in front of her to try to stop her. Unfortunately, she didn't listen as she pulls the female away from you. Rin then picks you up and held her arms around you to protect you.
Rin: I don't know who you are, but you step away from my brother!
Jack: Aunt Rin! It's us! Jack!
The adult female named Jack made you and Rin surprised and also confused at this.
Rin: Jack...? Wait, what..?!
Jack: Yes, it's us! We're Jack!
She exclaims with her arms in front of her in defense. You were about to speak up until Megumin, Aqua, and Darkness came into the room.
Aqua: (Y/N), what's going on!?
Megumin: We heard the noise and-
That's when they see the Adult Jack in front of them.
Darkness: What the?! Who are you!?
Darkness asks until you immediately got in front of the Adult Jack.
(Y/N): Everyone, wait! It's Jack! Hold on!
Aqua: Jack?
Darkness: This woman here is...Jack!?
Rin: Yeah, that's what (Y/N) and I heard.
Suddenly, Kazuma came into the scene.
Kazuma: Hey, what ha...oh my god...
Kazuma said with his face dropped into a surprised one before a light blush appeared on his face.
Kazuma: I-Is that...Jack...?
(Y/N): Alright everyone, don't do anything stupid! Jack, can tell us what happened to you!?
-Mini Time Skip-
(Y/N): Wait, your body changed while you were sleeping?
Jack: Yes, and when we woke up, we had these big breasts on our chests.
Jack says while moving her breasts up and down using her hands as you immediately look away while Kazuma stared at them.
Megumin: Kazuma! Don't look at them, you idiot!
Kazuma: Hey come on, I can't help it. Although, Jack is kinda hot in her adult form.
Rin: Wait, adult form?
Aqua: Did you do something to Jack's cup?!
The girls turned to Kazuma with angry and disgusted looks until Kazuma spoke up.
Kazuma: Alright, you got me. I purchased a potion at Wiz's shop and I decided to pour it into Jack's cup.
That's when you karate chopped Kazuma's head.
(Y/N): You fool!! What were you thinking!? You turned Jack into an adult!!
(A/N: You as Tanjiro while Adult Jack is Nezuko.)
Kazuma: Hey, I had to! I just wanted to see Jack if she was an adult, that's all!
(Y/N): Without my permission!?
Kazuma: Yes.
You face palmed out of frustration.
(Y/N): Goddammit, Kazuma.
Megumin: Well, since Kazuma had confessed. How long does it last?
Kazuma: Wiz said to me yesterday that if I used it and the potions will affect which is today and it will end in 24 hours.
Jack: So we have to look like this for the whole day?
Kazuma: Yes.
(Y/N): Great. Anyways, I'm not letting Jack go outside looking like that. It's...really revealing...
You said with a cringe expression.
(Y/N): Does anyone have any extra clothes?
Everyone shakes their heads in "No" as you let out a sigh before speaking up.
(Y/N): Great. I'll get you some clothes, Jack. Just wait here, okay?
Jack: Okay, mom. We'll behave.
(Y/N): Good. Rin, please keep an eye on her and Kazuma as well.
Rin: You got it!
Eventually, you sped away out of the mansion to buy clothes for Jack.
-Mini Time Skip-
Within a few minutes later, you bought some decent clothes for Jack as you used SuperSpeed to get back to the mansion.
(Y/N): I'm home!
Aqua: Whoa, that was quick.
(Y/N): Yeah. Now then...
You said as you went to your room and knocked on the door.
(Y/N): Hey Jack, I'm bought you some clothes.
That's when the door opens as you see Jack still wearing her revealing clothing. You blushed a little before handing her the clothes.
(Y/N): H-Here you g-go...
Jack: Thanks, mom.
Jack then takes the clothes before closing the door in front of you as you began to wait for Jack to change her outfit.
-Mini Time Skip-
For about a minute later, you hear Jack calling out to you.
Jack: Mom, we're finished~!
You let out a sigh as you opened the door and took a look at Adult Jack's clothes that you had bought.
(Y/N): So, what do you think?
Adult Jack moves her body around a little before she spoke up to you.
Jack: It's wonderful.
You smiled at her response
(Y/N): Glad you liked it.
Jack: Hey mom, can you...
(Y/N): Can I...what?
Jack: Can you sit on our lap?
You found this surprising.
(Y/N): Oh uh...sure thing...
You said as Jack sits down at your bed and pats on her lap, but she seems to be a lot taller than you, for around about 6'0.ft.
You sighed as you eventually sat on her lap. To your surprise, it was quite soft then it looks.
(Y/N): (Huh, it's not bad.)
You thought to yourself with a surprised face. That's when you feel Jack's hand stroking your head.
(Y/N): Um...Jack?
Jack: What's the matter? Do you not like it?
(Y/N): O-Oh no, no, no. I like it.
You nervously replied back.
Jack: Really?
(Y/N): Yeah, I've been patting you a lot so...this might return the favor to me, I guess?
Jack: That is nice to hear. Let us pamper you a little, mom.
(Y/N): Well...okay then.
Eventually, Adult Jack pampered you for quite some time and you feel like that you're getting spoiled.
-Time Skip to Evening-
Hours have been passed since Jack turned into an Adult. Both you and her have been going on a single quest and Jack was pretty good in her fight in her adult body.
So right now, it was nighttime and you decided to take a shower in the bathroom after a long quest. You sat down on the chair while using some Shampoo on your head until you suddenly hear the door opened as you turn to the door.
And to your shock, Adult Jack entered the bathroom without permission.
(Y/N): J-J-Jack!? What the hell are you doing here!?
You asked with embarrassment as Jack tilted her head to the side before speaking up.
Jack: We thought if we joined you in the bath. That's what any family members or friends do, right?
(Y/N): Y-Yes, but you're a child with an adult body! There's no way I can do that!
Jack: That's true, but we still want to take a bath with you. We can only clean your back or the front if that's what you want, mom.
You facepalmed out of shame because this is a really embarrassing situation for you to handle.
(Y/N): Alright, fine...but don't go trying touching my private parts, okay?
Jack nods as you let out a sigh. Within a few moments, you felt wet hands being rubbed against your back. This startled you a little which alerted Jack.
Jack: Are you okay, mom?
(Y/N): Uh y-yeah...I'm fine...just startled, that's all...
Jack: Okay, let us continue.
(Y/N): *Nods* Mm.
Jack then continues cleaning your back using her hands before she used a wet towel to rub your back.
This goes on within for about a minute until she spoke up.
Jack: Do you want us to clean your front body?
(Y/N): U-Uh...y-yeah, s-sure...
You replied back with an embarrassed look on your face.
Jack: Okay then.
Jack then started to rub her hands with soap as she began to clean your front body. Suddenly, Jack started leaning your body and you can feel her large breasts touching your back.
You were clearly screaming on the inside because of a female child with an adult body cleaning your body.
This goes on for about a minute and a half until you eventually calmed down a little.
Jack: Are you enjoy this, mom?
(Y/N): Jack, saying like that is gonna make it more embarrassing, you know?
Jack: Oh...We're sorry...
(Y/N): It's fine. Anyway, I think it's should be enough.
Jack: Alright.
-Mini Time Skip-
After taking a bath, you were sleeping on your bed with your pajamas until you felt someone's hand and they started shaking you gently.
You slowly opened your eyes and see Adult Jack looking down at you.
(Y/N): Jack? Why are you up this late?
Jack: We had a bad dream, mom. Can we sleep with you?
(Y/N): Oh...uh...sure...
You then move some space for Jack to lay down on the bed with you.
Jack: Thanks, mom. We feel comfortable now.
(Y/N): Yeah, no problem.
You said with a light blush and a smile on your face because of Adult Jack's beautiful face.
Jack: Umm...can we hug you, mom...?
(Y/N): Well...alright.
Jack then wraps her arms around you as she moves her body close to yours. You were a little embarrassed because her breasts were on your face.
Jack: We like this. Goodnight, mom.
Jack says with a smile on her face before closing her eyes.
(Y/N): Yeah...Goodnight, Jack.
And just like that, you closed your eyes and you and Jack drifted off to sleep.
-The Next Morning-
The Morning has now risen from the horizon as the ray lights started shining on your eyes, blinding you a little.
You slowly woke up from your sleep as you can hear light snores from your left. You turn to your left and see Jack was back to her child self and you can see that she was hugging your left arm.
You smiled that she was now back to her normal again after one big sleep as you started patting her head.
(Y/N): Wake up now, Jack. It's already morning now.
Jack: Okay, mom...
...
(A/N: That's that~! Sorry for not posting another chapter for about two weeks. Now that's done, let's head to the main story~!)