Tsundere! Prince! Tribe Sans x Castaway Reader

I know that there's an au like this named tribaltale. You can use their designs but I'll change a lot of things since I don't even know a single thing about this au. Anyways, a little info, the island that the monsters are in is surrounded by a barrier which you can get into but you can't get out. They all live in tents. Also, sans is a bit stubborn and... Well, I don't really know how to describe his attitude but you'll see later on. This is also a soulmate kind of AU where you and your soulmate have a mark. Sans will take time to be a tsundere in this one sooooooooooooo.... You have to be patient.
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Your pov

Finally! You can finally go to (insert beautiful country here)

(E.g: Hawaii or something. I don't know all of the countries and how beautiful(?) they are)

You boarded the boat as an attendant(?) of the boat carried your things. You ran to your assigned apartment as your grin stretched to the point where it came up your ears. Well, not literally but metaphorically. Your things got dropped off in the middle of the room as they left after you gave them a tip. You examined the room. The room has a king sized bed, a window, a mini fridge, table, lamp, a fancy looking chair, a huge, soft and silky mat, cabinets, a modern kitchen, drawers, bookshelves, an air conditioner, a fancy trash bin and a few potted plants all around. Over all, the apartment was huge and beautiful. You closed the door and ran around, gawking over a few things. You looked out the window and saw that the boat was about to go. You heard it honk as it went out into the vast ocean. You looked back at the land for the last time before it disappeared into the distance. You sighed and closed the window's curtains. You took off your footwear and jumped into the bed. You snuggled into tbe soft covers, sighing contentedly. It was one of the softest beds that you have ever laid on. You hesitantly stood up and went to the mini fridge. You opened it and saw a pitcher of water, some eggs, a milk carton and a chocolate bar! You took it and sighed in relief when it wasn't expired yet. You unwrapped it and chewed on it contently. You closed the mini fridge and decided to fix your things.

Sans' pov

I sat on the throne next to my younger brother, papyrus, who was next to asgore and toriel's bigger thrones, as I zoned out. I wanted to take a nap but pap won't let me. Geez, why do I have to be a major part of the tribe? Why can't I just be a regular monster? I groaned quietly as I listen to our alpha, asgore, ramble on on something I didn't want to know. I was so bored! "...and we will make our beta the new alpha!" Cheers erupted from the crowd below. I stood up straight and almost did a spit take from shock. What? Well, let me get this straight. Since our dad died, I was promoted to beta since I was the eldest son. I didn't think that I can be promoted into this! Asgore looked at me making me come there. I went towards him as the cheers for louder. I forced a smile on my face and kept my back straight. I dont want to but asgore would probably bug me about it as well as papyrus. Toriel, our second alpha, looked at me. "Of course, as the tradition of our tribe, you must pick a girl alpha to rule with you." I flinched. I forgot about that but, why the heck would I even need another alpha, a girl nonetheless? I could handle myself just fine without a girl sitting still beside me, just looking 'pretty' and acting like she owns everything in the world.

(Sorry for offending some girls out there. I even got offended. Geez, thanks sans. Thanks (sarcasm).)

After the ceremony, I was about to go back home but a hand stopped me from doing so. I turned around and saw toriel once again. "You cannot go to your home just yet. You still need to learn about some things." Great. Just what I 'needed'.

Your pov

You sat at the dining table, eating away. So delicious! You can't wait for dessert! You practically inhaled your lunch and asked for seconds. You can't help it, it was delicious! And plus, it's free food! Who can't resist free food? The only things you paid for was the trip, apartment and service so there wasn't that much problems. You finished it in mere seconds. You stood up, completely full. You then went to the front of the ship where you can view the sea. Luckily, you didn't really get sea sick. You sat at the empty seat, which was close to a tank of some sort. You sat on it and relaxed as the cool wind hit your face. The clouds were a bit dark though. Suddenly, it rained. It was a storm! You stood up and held to the bars at the edge of the boat as tight as you can. The boat rocked left to right, the waves crashing the bottom of it. You were wet and shaky as you tried to go back to the inside of the boat but it rocked so hard that you flew to the outside of the boat. You screamed, it being barely heard because of the storm. Your grip was slipping, seeing as the bars were getting more slippery. Nobody heard your pleas as your grip grew more loose. Suddenly, a stray rock that was sticking out knocked the boat with a lot of force. You lost your grip and let out your last screech before falling into the wave's unforgiving grasp. The impact was hard. You sunk down at the sea. Your eyes closed as you fell unconscious.

Sans' pov

We were at the beach part of our island. Toriel and asgore were training me. They said, as an alpha, I need to be strong to defend the whole tribe. Sure, it is needed but I hate training. I fired a few bone attacks at the dummy, injuring it quite badly. This is the 15th time and I had to 'perfect' the attack. Asgore looked at me, looking a bit more satisfied. "The attack is alright for now but you have to work on your attack damage. I recommend taking a few pills that dr. Alphys will make but that can be worked on later. Come on, we have to work on your aim next." I sighed silently, trying to feel the breeze for at least a second so that I can cool down. I didn't even sign up for this! I followed asgore and I saw bottles in the distance, at least 5 meters far. "Alright, now try to-" "A human!" he got cut off by toriel. I looked behind me to see toriel kneeling down at someone, most likely the human, and inspecting them. Asgore and I walked towards them. I looked down at her, judging by her looks, unconscious body. Asgore thought for a moment. "help me bring them to alphys." The words spilled out of my mouth before I can even think. They were shocked by my suddenly command. "now." I carried the human as the two goats followed me. What am I doing? I'm supposed to kill this human to break the barrier that is trapping us in this island! That thought was ignored when we arrived. "alphys!" I called out. The yellow reptile quickly made her way towards us. "Y-yes a-alpha s-" she gasped at the sight of the human. "heal her, now." She nodded a little hesitantly and brought her to a bed, also known as some leaves and cotton that is a nest actually. Then she checked for injuries and got some herbs. (Since they are in an island, they use herbs.) I sat down on a chair and the retired alphas did so too. They looked at me in confusion. "Sans, if I may ask, why do you want to heal the human?" Asgore said. I looked at them, a bit troubled in terms of finding the right words. "i-i don't know. i just felt like i had to. like, when i first saw her, i just had to help. don't know why though..." Toriel gave me a knowing look as asgore looked at her weirdly. Toriel whispered something to him and he nodded with a mischievous smile. I sighed at their mysteriousness, quite annoyed. "continue being the weird couple you are. i'm going to see the human." I stood up and went to the nest which was just in the next room. I almost bumped into alphys. She looked at me, a little surprised. "A-ah! A-alpha sans! I-i'm so sorry for almost b-b-bumping into you! A-anyways, I-i was about to t-tell you that the h-human is s-still healing. T-they are just unconscious and they w-will wake up l-l-later on." I nodded as she fidgeted with her clawed hands. "alright, thanks for the info. go on, i'll stay here." She nodded and went off. I walked over to the nest and sat down next to it. I observed the human. She had (h/l), (h/c) hair. It was messy and wet. She wore clothes that a tourist would wear. I looked at her face as blue tinted mine. She looks so peaceful. My soul skips a beat. Wait... I shook my head and moved my head away from hers. What am I doing? I shouldn't fall in love with her, especially since she is a human. Besides, we just met! But my soul can't help but call out for hers. I gripped my chest as I growled, my mind having a mental debate about the feeling. It was cut short when I heard shuffling. I looked down and saw two, tired, (e/c) eyes staring at me. It widened as the girl shot up, crawling away from me. Her scared expression made my soul sting a little. "hey now, don't be afraid, i won't hurt you." I tried to give a comforting smile. What am I doing? It seems like my feelings had complete control of my body. She looked at me as we stared at each other in silence. I saw a faint blush on her cheeks. I felt my soul flutter. What the heck? I did my best to ignore the feeling, seeing as it kind of annoyed me and it will most likely creep her out. Wait a minute, why am I worried about her feelings? Why am I like this?! I didn't notice that I was spaced out until she spoke, her blush more evident. "U-ummm... Are you okay?" I shook my head and looked away with a scowl. "you'll be staying here with us. asgore and toriel will lead you to your tent." I walked away, an annoyed aura around me. "W-wait!" I ignored her as I went into the waiting room. "toriel, asgore, can you lead the human to an empty tent? i'm going to do something." I spoke before going out, not even waiting for their response. I just need some fresh air. I took a deep breath while I was walking. I didn't really have a specific direction in where I was going but I know that I won't be lost, seeing as I know this island like the back of my hand. I was distracted by my thoughts. Why am I like this? Why did I even let that human live? We should've been free and I could have been free from all this 'ruling the whole tribe' mess. But... The thought of killing her..... saddens me.... Why though? I sighed as I looked at the mark on my wrist and traced the mark on it as I sat down. It was a circle with a gray heart in the middle. A line was splitting the both of it apart. Though it seems like it's a big mark, it's not really that big but it's not that small either. I covered it again with the thin leather once covering it. I should really go now, they might be waiting for me. I stood up and looked up and saw the sun setting. I actually sat there longer than I thought. I snapped my fingers and teleported back in me and my brother's tent. The smell of spaghetti reached my nose. I walked over to the kitchen part of the tent. Papyrus, hearing my footsteps, turned around and smiled a big one once he saw me. "BROTHER! WHERE HAVE YOU BEEN? I WAS SO WORRIED ABOUT YOU!" I smiled. "i just went to train. asgore said that i have to. i also found a human." His eye sparkled once I mentioned about the human. He always wanted to see a human. Even though he was also beta like me, I was the one who did most of the work involving humans, afraid that he might get hurt. "CAN I SEE THEM?! PRETTY PLEASE!" His voice snapped me out of my thoughts. I hesitated. Should he? Maybe she was still a threat. I shouldn't really trust humans but, something about her makes me want to trust her. She seemed harmless. I'm not too sure. I sighed. "sorry paps but you can't." He frowned a little. "WHY NOT? I'M SURE THAT THEY WOULD LOVE TO SEE THE GREAT PAPYRUS!" I laughed a little but I looked at him apologetically. "no. you just can't. we're not sure if they are a threat or not. i don't want you getting hurt-" "BUT WHAT ABOUT YOU?" I got cut off. "WHAT IF THEY ARE A THREAT AND THEY MANAGED TO KILL YOU? WHO WOULD BE THERE TO SAVE YOU?" I tried reassuring him. "pap, i'm not going to get hurt that easily. i can defend myself. that's why i train with toriel and asgore, to defend everyone and that includes myself." "I KNOW BUT SANS..." he looks at me seriously. "Don't Forget That You Only Have 1 HP. If They Strike You Once, Your Chance Of Surviving Is Quite Literally Non-Existent. I-I Don't Want You to Die." He looked to the side, slightly tearing up. Well, he did have a point. Besides, I can't dodge forever. "SO, TO ENSURE YOUR SAFETY, I WILL COME WITH YOU!" I sighed, giving up. My smile turned into a worried frown. "alright bro. just, be careful alright?" He nodded. I started to head to the table behind him. "get ready tomorrow. we must be early." "ALRIGHT BROTHER!" I sat down at the chair as papyrus brought the, thankfully not burnt, spaghetti to the table. We both ate in silence.

The next day....

Your pov

A goat couple, who's names are toriel and asgore, leaded you to a tent yesterday. You got a bit comfortable after they gained your trust. They told you all about their island and who the rulers are, why they are here, etc. etc. Back to present time, you sat up as sunlight seeped through the leather tent's opening, also known as it's door. You yawned and rubbed your eyes. You sighed as you felt and smelled the fresh air. You still have to get fully used to waking up in a nest instead of a bed but hey, it's soft so you can't complain. You stood up as you saw someone enter your tent. You saw sans, the island's alpha. There's also another skeleton that followed him. You assume that he is papyrus, the island's beta, also known as sans' younger brother. Sans looked at you with a slight glare but yet his cheeks were tinted blue when he saw you. Papyrus, carrying a bowl of spaghetti sided with fruits, smiled brightly at you. Not really the best combination but meh. "GOOD MORNING HUMAN! I, BETA PAPYRUS, AND MY BROTHER, ALPHA SANS, HAS BROUGHT YOU SOME FOOD TO EAT! WE HOPE YOU ENJOY YOUR STAY IN THIS ISLAND SO FAR SINCE YOUR NOT REALLY GETTING OUT OF IT SOON." wait what?! You looked at him with wide eyes while he handed you the bowl. It turns out that you said that loudly. Sans responds. "yes, you'll need to stay here until another human gets here. that way, we can use their soul and the other humans' souls that we collected throughout the years to break the barrier trapping us in this island. didn't our former alphas told you about that?" Oh, right. You chuckled nervously. "Ah, they did tell me about it. I just forgot about it... Sorry..." You blushed a little, scratching the back of your neck while looking away. You saw sans' skull turn into a light shade of blue in the corner of your eye. When you looked at him, it quickly disappears, well, most of it anyways, and he looked at the side, furrowing his bone brows. Now that you think about it... "If you needed a human soul to break the barrier, why didn't you take mine earlier? I mean, you guys have the chance right? Why spare me?" Sans' blush increased. Papyrus looked at him. He then gave him a knowing look. Sans glared at papyrus for a moment. "u-uhhhh.... i-i.... umm... i just don't want to kill you alright?" You find it a bit suspicious but you let it slide, for now...

I'm sorry but I'm splitting this in two parts. I have to think of how this will turn out and well, it's already long. Not that it's the longest one I made but hey, it's already 2000+ words and yet you and sans are only meeting each other yet. Anyways, boi p chips! And also happy advanced new year and merry late Christmas! :3 (I forgot to make a Christmas special hhhh- I'll make a new year one, don't worry ;3)