W. D. Gaster x Reader

I'm Not A Boy: Waterfall

I grew up tough, like a tomboy, only I used to make people honestly believe I had a penis even though I have a very feminine voice. So, I learned to mask it, sound like a boy. And it wasn't like I wanted to be a boy over being a girl, I'm fine with my gender and my body, I just wanted to appear tougher. I worked out, got stronger. Of course, working my legs and arms only enlarged the muscles in my breasts and bum, so those only got bigger and tighter. But still, baggy clothes hide it well, especially if I wear XXL, then I can really look like a guy. A huge guy, but a guy.

I remember meeting Alphys and Undyne, for the longest time they thought I was a boy. sans and Papyrus, I'm not sure they even know I'm a girl. But Gaster, I don't think I've ever directly told him I'm a girl. He might not know. Which is hilarious since we share a house in Waterfall, we became good friends when we met and Waterfall is the only habitat I can even survive in. So it seemed like a good choice.

Today, we invited sans, Papyrus, Alphys, and Undyne to a little swimming party in the Waterfall. We just thought it would be fun to swim. Of course, I was really nervous. When it comes to certain occasions, I would rather dress like a girl. For a formal event or to go o a club I'd wear a dress, and for swimming I have a bikini. But, I always get nervous, like being girly makes me weak. Honestly I don't enjoy dressing like a guy as much as I used to, I've started to think about dressing like a girl more and more. And of course, I get nervous I'll be judged and hurt and labeled because of it. It can be the worst thing in the world to be a girl.

I had my bikini on, my hair was out, usually it was shoved in a hoodie or a hat, but today I just had it down and flowing. Looking in the mirror, I realized I am beautiful. I look almost like how goddesses in movies and art look, but better in my opinion. I heard a knock on my bedroom door, "(Y/N), it's Alphys and me, uh, what's taking so long?" Undyne called. I opened the door with a blush. "I'm just ..... shy." I said and they gasped when they saw me, they both blushed. Alphys was wearing an adorable black and white striped one piece that looked like a pair of shorts and a tank top put together. Undyne had on a strapless blue bikini top that covered her up well and a pair of matching blue shorts.

"How are you two so confident? I mean, I can barely act like a girl without feeling, weak and defenseless. But you two, you're just so brave! And I hide behind this boyish disguise and, you two are strutting in your bathing suits. And I can't even make myself wear a bikini in front of people." Halfway through my voice picked up into my normal feminine voice. They seemed just as surprised over that. "I can't even act like myself." I sighed and Undyne was quick to jump in, "Hey, don't bring yourself down. You go out there, and you show them that being a girl is fucking awesome! You show them that yea you're a girl, but you fucking rock! I promise you, no matter what you wear, as long as you're being yourself, you're at maximum strength!" I smiled at this, "Okay. If you really think so."

I took a deep breath and went out there with them, Papyrus turned to greet us, but his cheekbones turned orange and his lower jawbone fell off into the water. I gasped and went over to him. "Are you okay?!" I bent down to grab his jawbone and I helped him reattach it. "Oh, yes! The Great Papyrus is perfectly well! I only, uh, was surprised by your lack of clothing. And uh, such....." "You thought I was a boy." "Indeed." "Well, I'm a girl." He gasped, "And all this time I've failed to treat you like a proper lady!" I giggled at this, "Don't worry about it Papyrus." I said, punching his arm. "Ouch."

sans turned at the sound of his brother letting out the noise of pain. His eyes widened when they landed on me, his cheekbones went blue and he just turned away, "*uh, nice of you to join us." "Yea, nice to be out here bro. I haven't gone swimming in a while." I started to make my way over to the waterfall. I made my way to a rock and I sat down, watching the waterfall from where I sat behind it in the little cave. I then heard talking, "Where's (Y/N)?" Gaster asked, "I believe she's behind the waterfall." Papyrus said. "She?" "Yes. She." Oh great, Gaster also thought I was a boy.

I stepped through the waterfall and I greeted him, water running down me. "Hey Gaster." I said in my normal, feminine voice. "It's me." I said again in my deeper male voice. But honestly, it didn't honestly sound like a real boy's voice. I giggled in my regular tone. His cheekbones went purple and his jaw nearly fell into the water. He quickly fixed it. "Uh, my apologies man, ma'am. Uh." I smiled, "I'm a girl." "Right." I sighed some, "I get confused for a boy all the time, no worries." "Had I known you were a girl I wouldn't of let you hang out in my room with me while I changed clothes all those times." We began to walk, climbing out of the water to continue our conversation.

I smiled, "Oh, believe me, I didn't mind the view." He blushed darker, he cleared his throat. "I guess that would explain your name, (Y/N). I don't know much about human names but it did sound sort of feminine." I nodded and I gave Gaster a smile. "Geez I can't believe I thought you were a boy." I chuckled, "Most people are surprised. It's perfectly fine." "Yea....." he drifted off some, staring at my body. He must like what he sees.

I smirked and I put my hands on my waist, I knew he wanted me, his eyes lightly glowed a dark purple. It was starting to become my favorite color. I turned towards the water, "Wanna take a lick?" His eyes widened and became more saturated, "What?" "I asked if you wanted to take a dip. Ya know, get back in the water?" He clenched his jaw some, on the verge of self-control, nearly slipping off the edge and losing it. "S-Sure."

We hung out in the water, but after a while Gaster said he was tired, so he went inside. Once everyone else left to head home, I went back inside. "Gaster? Everyone went home. You asleep?" I asked as I looked around for him. I froze when I heard a low moan. I wasn't sure where it came from, but, I recognized it as Gaster. And for some reason, I felt heat between my legs. I blushed and I listened as the moans continued, soon I found myself outside Gaster's door, I kept a distance so he wouldn't see my shadow under the door.

"Oh fuck, (Y/N)!" Was he seriously fantasizing about me? Well now I HAVE to listen in!

I blushed as I listened to him, "Mm, I'm gonna fuck you so hard! Tricking me, mmm! Lying to me! Teasing me all day with your wet body! Aaah!" I squeezed my thighs together, squirming at the feeling. "Oh! You've been such a naughty girl! Mmm, naughty girls must be punished." My eyes widened. He was fantasizing about sexually punishing me. I blushed darkly, what is he thinking of doing to me? I cupped myself, desperate to relieve the pressure between my legs. My hips began to buck as my mind wandered with the possibilities. I quietly made my way to my room.

Now I have a serious problem between my legs and the only thing I wanna do is hold on to Gaster's shoulders, bury my face in his chest, and bounce and grind on his lap.

But, I would never be brave enough to actually do it.