When you go down, you go down hard.
Knees first, then onto your hands. Every last fiber in your body cries out when the full brunt of the forward-fall bruises your legs and tiny, sharp shards of pebbles embed themselves into your palms from where you caught yourself. The back of your throat burns with every desperate intake of breath, chest aching and heaving with effort.
"That's... That's just pathetic!" You hear Undyne spit at you, her feet grinding to a halt nearby. You stare at the ground, bereft for response for the time being. "I'm sure even Sans can run more than that. Are all humans like this? And after you've been training with me for so long?!"
You can do nothing but swallow air, attempting to stabilize yourself. She's at a standstill and probably waiting for a response: you can practically sense the impatience radiating off her.
Confirming your suspicions, you feel a hand close in on your back, roughly seizing a handful of fabric and yanking you up to your feet by your clothes. The familiar face of the piscine woman greets you as she leans in close, inspecting you. Her mouth is curled down in an annoyed grimace, her oversized, yellowed razor teeth jutting out over her lower lip as though her mouth were all too crowded to contain them. After a moment, her visage softens and she blinks at you in obvious concern.
"H-hey, say something. You're not dead, are you? Are you dying?"
"I'm not dying, Undyne." You assure her, your breathing and heartrate steadying to normal. "I just feel like I am after that session."
"Aw, wimp." She reverts back to her usual self instantaneously when you respond, releasing you and permitting you to stand alone. "Don't worry me like that ever again. Well, whatever, I'm sure you'll do better next time!"
You give her an incredulous look and take a moment to reflect on the marathon training session you'd just completed, and rather competently at that, after being forcefully dragged off the skeleton brother's couch by both Undyne and Papyrus to join them on their morning run. You feel you did better than she gave you credit for. In fact, at this point in your training (and after sparring with countless other monsters), you have a vague awareness that you're fitter than most of your human counterparts on the surface by now.
"Hey now!" She scolds, reading your expression, "Don't get all sour on me just because you weren't perfect this time."
You sigh, understanding that there was no bargaining with Undyne on such matters. You simply assure her that 'you'll do better next time' (she leers at you when she detects the note of sarcasm) and swipe your forearm against your sweat-sheened forehead. It's simply best not to engage her on that line of conversation. You look forward to your post-workout bath.
The cool water is a merciful bliss, wicking the heat away from your skin as you slide into it.
Sedately relaxed, you focus your vision just enough to admire your surroundings. Dim light radiating from the bioluminescent flora carpeting the walls and riverbed floods the area. Close by from a higher plateau, another river crosses the precipice and flows into this, creating a pristine waterfall. You'd come to this secluded spot often, usually to bathe following your intense sessions with Undyne, but sometimes merely for a moment of repose and solitude. You mentally note that you'd followed the stream before and seen that it's a smaller, hidden away trickle that eventually fed in to the main river running alongside Waterfall, and then to Snowdin, the town you called home for the time being.
Distinctive sounds of activity prick your hearing and your eyes immediately snap open, body alert and poised to move. You scan the horizon for the source of the noise and your gaze falls to rest on the opposite side of the river, some ten meters away, where a very familiar figure was tearing her clothes off with relish.
Without a moment's hesitation, you plunge yourself nose-deep and crawl to a nearby outcrop of rocks, concealing yourself as best you could behind them. You keep yourself discreet and low, peering out from behind your cover as Undyne finishes rending herself bare, wading into the river and moving toward the waterfall, where she stands and allows it to crash over her.
Reams of cool riverwater trickle down her athletic form, playing over the rippling musculature of her back. Iridescent aquamarine scales shift between various hues of blue and green in the limited light as she moves, accented with glittering droplets. Her scarlet hair is let down for once, unkempt, damp and adhering to the curvature of her neck and shoulders.
After a few, tensely prolonged moments, she appears to turn her back to you fully, busying herself with rinsing her face. Your gaze darts off to the nearby riverbank and you mentally rehearse your next movements: one, slink off to the riverbank as quickly and quietly as possible. Two, snatch up your pile of clothes. Three, leg it.
You manage to stand without making too much noise -- you figure the crashing water around the waterfall was sufficient to drown it out. Doing your best to ignore how exposed you feel, you move carefully toward the bank, the resistance of the river pulling at your ankles with each calculated stride. You scramble up the short bank, keeping your body in stoop in preparation to snatch your clothes and run--
Your stomach pits with dread when Undyne's bellowing voice cuts through the air, leveled at you, immediately followed by sounds of rapidly-approaching, churning water. You spin around, ready to rapid-fire apologies and explanations, but Undyne's already closed in on you, forearm on a trajectory to bash you in the chest.
The force of the blow to your upper body sends you toppling. Earth and rock is harsh against your naked back when you connect with the ground, causing you to screw your eyes shut in pain. A flat, webbed foot stamps down on your chest and commands you to stay in place as Undyne shifts to place all of her weight on it. The pressure draws the air from your lungs.
"D-disgusting." The patter of droplets falling onto your skin wrenches you back into reality and you finally dare to crack your eyes open. "After all the time we've spent together. I never thought you'd be a dirty peeping tom." You summon every ounce of your willpower not to stare at the most private thing that hung so prominently over your field of vision from this angle and instead lock your eyes with her amber orbs. Her expression is ferocious, but the ruddy tinge around her cheeks and the slight softness to the corners of her almond-shaped eyes betrays her feelings of shame.
"Undyne." You start, voice stern. "I'm sorry, I--"
"I don't want your lame excuses!" Undyne's chest flares with a sudden burst of soul magic as she reaches into it, producing a long spear of energy with one brisk sweep of her arm.
"No, I'm being serious, it was just a bad coincid--" Your teeth reflexively wire shut and your voice ceases when you feel her nuzzling the spearhead against your jugular.
Then, she simply stands there in silence, keeping you flat to the floor with her weight and an edge at your neck. Her mouth hangs slightly open as though she's considering something deeply.
"You know what? Fine, then. Since you wanted it so badly, here. Have a look. Drink it aaaaall in."
"Undyne, you're still misunderstand-"
The pressure of the spearhead increases, forming a perfect, straight dimple against your throat. When she speaks, her voice comes out as a low growl.
"Look. At. It."
Permitted now -- forced, perhaps -- your gaze wanders the form of the monster woman looming over you. She's still dripping wet from bathing moments ago. Minute rivers run the crevices of the valleys of her chiseled abdomen, flowing down into twin rivers either side her mons pubis, then trickling off her thighs and onto you. Her slit is tidy and modest, her inner lips a deep, sapphire hue, a few shades darker than the rest of her.
"Oh, so you do like what you see." Your eyes dart back up to her face and find her smiling wickedly down. It makes you wonder what involuntary response she might've seen from you.
The fist holding the weapon uncurls, the spear dissipating into nothing. You breathe a sigh of relief at the sight of the polearm dissolving in your periphery. The movement of your chest reminds you that her foot was still rooted there on your sternum. The weapon was gone, so why wasn't she letting you go?
Undyne gives you no time to process the turn of events as she drops suddenly to her knees, legs splayed either side of your head, pussy perching itself squarely on your mouth. You let out an involuntary yelp of surprise into it.
Wide-eyed and not quite grasping the situation yet, a questioning sound burbles from you.
"You were slinking around here to catch a glimpse of me." It's disconcerting to hear her speaking so seriously for once. Her rough, clawed hand reaches down to rake through your hair and seize a handful of it, yanking painfully from your scalp. "I'm just making us even so I can forgive you. Subjecting you to something you didn't ask for, making you taste your own medicine. Turnabout is fair play, kid."
Her weight bears down on your jaw, giving you no option to back out. Tentatively at first, you poke your tongue out and give her slit a brisk, broad lick. She responds by pulling your hair tighter, turning her claws away from your scalp as best she could, murmuring a satisfied 'mm'. She yields slightly, holding her own weight and giving you a bit more berth to move around and work.
You'd often thought about how Undyne's aura is vaguely reminiscent of sashimi, but now, buried nose-deep in her womanhood, the comparison seems more apt than ever. The appetizing aroma of fresh seafood envelops your senses, completely stifling the air between her thighs. When you run the flat of your tongue over the soft mounds of her smooth outer labia, it conjures vivid images of cuts of succulent fatty tuna. When you scoop a sampling of her warm, feminine juices with your tongue, the slight sour tang of the fluid is like the finest rice vinegar.
"Nngh... good... more."
A surge of panic runs through you when you feel her finely-toned thighs compressing tighter around your head, muffling your hearing.
Your hands move to caress the backs of her scaly legs while you work. You press the tip of your nose into her bud and split her with your tongue, thrusting it as far into her as you were able. Her honeypot responds to you in equal, enthusiastic measure, gripping at the edge of your tongue and dripping steadily into your mouth, rewarding you with more of that heavenly nectar. Hearing hampered and vision mostly obscured, Undyne dominates your senses. You become hyper-aware of her responses and the facets of her body. You feel the thin, mucosal film coating her body as your hands glide affectionately over her scales. You sense her muscles seizing and relaxing in response to your ministrations, responsive and pliant. You hear her breathing become more heavy, more labored, more ragged.
"C-come on. I said more. Give me more than that." She tries to sound authoritative, but half-wrapped in a lustful haze, her voice lacks some of its usual commandeering edge.
Giving a small nod, you scoot your body back a measure, opening up a space below your chin. You know she needs something more filling, more substantial.
You dare to tease at her opening with the pad of your index finger, tracing the rim of her slickened entrance in small, slow circles. You can feel every fiber of her being quivering and quaking with anticipation, body begging for release though she'd be too proud to ever admit it vocally.
You knit three fingers tightly together, pushing them just past her lips to align them. Then, heedlessly, you plunge into her depths with a sharp push. She finally cries out, unrestrained and voice dripping with ecstasy, hot, textured passage clamping down around your hand like a vice. Your eager tongue slathers her clit as you begin to move your wrist in rhythm, pushing up and into her, the force of it bucking her whole body. Sweet, wet pops sound from her hungry cunny as it strains to grip you, her voice chiming moans from above, just out of sync.
Not wholly satisfied with your pace, she rolls her hips experimentally, driving your fingers deep into herself. You arch your neck in a valiant attempt to keep up with her increasingly rapid, erratic movements, fixing your mouth on her swollen pearl, teasing at the trapped, tortured nub with the tip of your tongue.
Suddenly, Undyne's body doubles over and the spasm of her hips goes sloppy, grinding her quim clumsily against your face, smearing her essence over your nose and mouth. You let her ride it out, gripping the backs of her thighs as her body undulates and writhes. Your stomach clenches with an implacable, intense feeling when you hear her breathe your name in the heat of the moment, barely above a whisper, voice breaking.
You both sigh in unison when she makes a move to sit upright, lifting herself off you. Glistening threads of cum and saliva momentarily form a bridge between your mouth and her pussy before they break away, casting an air of finality over the scene. Undyne dismounts you and rolls her body aside, collapsing onto her back.
For a while, the sound of rushing water and the awareness of her presence nearby is all that exists as you lay there together in silence. Feeling it all too surreal, you bring a dirt-covered hand to your face and touch the lingering wetness she'd left on your cheek, just to prove to yourself that it was really there.
"Guess we need to wash up again." You remark uselessly, rolling your head onto your side to chance a look at her.
She laughs a little, snorting through her nose and dissolving the tension. She pushes herself off the ground and sits upright, shaking dirt and soil from her back. "Yeah! Guess so. C'mon, before anyone else shows up."
"By the way..." You fix your eyes on her, watching as she rises to her feet, interlacing her hands and stretching them over her head. The posture thrusts her chest forward in an artistic kind of way, though her breasts were modest in size. "That was pretty good. You're officially forgiven."
Before you can reply, Undyne performs a frankly impressive leap, clearing the bank and landing in the river a few reaches away. She drops to her knees and barely manages to submerge most of herself, her long hair floating to the surface in a splayed, crimson halo. She remains there for quite a long time, reminding you that she was probably capable of breathing underwater.
You finally pick yourself up off the ground and sigh, smiling to yourself. It was time to get clean again.