The Best Way to Earn Someone's Trust is to Drink Their Spit

Mischief and you normally went hand in hand, but since befriending Susie, that all changed. It wasn't that had you stopped at all, just that you and her had taken it to places out of the view of others.
It started with awkward flirting practice after an excursion in the dark world before turning into true awkward flirting and into awkward kissing practice, something which you still weren't sure who's idea it really was, but it further escalated from there. The two of you were finding excuses to sneak off at school, meeting up after hours, and breaking curfew to spend time alone in the woods, all for 'practice' make out sessions.
Susie was definitely responsible for what happened next. In the empty classroom, after having had her fill of your mouth, the reptile decided to skip ahead several steps and go right for your dick. Even though she was the one in control, her technique needed work, but it was plain to see she was trying her best. Things literally snowballed from there when after blowing your load, Susie, instead of swallowing, locked lips with you and spat your own cum down your throat.
Yeah, it was kind of gross, but you fired back by providing an even more enticing situation.

"A sleepover?" Your mother looked befuddled. You never were the social type, so she didn't know what to say initially. "Well, I suppose you can. Your friend can sleep in Asriel's bed just as long as they remember to tidy it up in the morning."
Susie dashed right over as soon as you called her with the news. Toriel had met her before, having been unsure of what to make of you being friends with the girl her students were frightened of, but managed to warm up to her. She didn't really like how much food she ate whenever she visited, though.
After a late dinner, the two of you retired to your room. After trying and failing to get past Asriel's password screen (you had already tried every permutation you could think of for "God of Hyperdeath"), Susie bodyslammed into his bed in annoyance.
After a moment Susie spoke up, realizing something. "Hey Kris, this bed is closer to your mom's room, right?" You nodded as what she was getting at dawned on you. She grinned. "Clothes off, then."
You do as you're told as Susie disrobes to just her bra and panties. "Heh, you look disappointed." She sits down and pats the bed, expecting you to sit with her.
You take your place beside her, taking in the darkness of the room, where the only light sources are Asriel's computer and some moonbeams from the window.
And with no warning, Susie proceeds to take your shoulder and grabs your dick. She pulls you back onto the bed and begins rubbing.

The feeling of Susie's scales... skin? Body? Whatever. Feeling her pressed against your own body like this is something else.
She's so preoccupied with her rubbing she accidentally leans too far back and bumps herself into the wall. "Ow, shit!" you hear her say aloud. You worry you may have gotten your mother's attention too quickly.
You hold your breath to attempt to listen in to any sound coming from the other room, but you hear nothing. Jumping the gun, Susie picks her handjob back up again.
Angling herself better against the wall for better leverage, her hand moves faster on your dick.
Almost forgetting the dangerous situation you're close to being in, you climax, your seed shooting out onto the carpet.

With the dreamlike weightlessness of your post-orgasm haze setting in, you fall asleep.
You wake up an unknown amount of time later after feeling Susie hands on you again. "You looked so cute, I almost didn't want to disturb you. Almost." You figured she'd want to do more.
You ask her what she has planned next. She wordlessly grins. That's disconcerting.
She repositions herself against the wall, pressing her snout right into it and pushing her butt out.
Hesitantly, you move yourself against her, knowing exactly what she wants you to do.
But which hole, though? She might get you back for it, but the idea of surprise anal puts a smile on your face.

With bated breath, you lift a cheek. With no light to guide you, you make a guess.
Susie's hands clasp over her mouth as she unleashes a noise you've never heard before. "$#?@%!"
You seem to have guessed right.
Not discouraged by Susie nearly waking your mom up with a probable tier 3 swear, you proceed with pumping her ass.
She hasn't said anything about you doing otherwise, so you must be doing something right.
Leaning in, you can hear her whines and... profanities escape her covered mouth.
And, well, you're already close as you can be for this, so you wrap your arms around her. Now you're her jetpack. Not that she'd care about that with what's going on now.
There's nothing else to distract you anymore, so you just thrust.
Eventually you can't help but cum in Susie's ass. The panting, sweaty mess that is the both of you separates as you take your dick out.
Just then you feel Susie moving to face you and pulling you into a hug. Guess she liked it after-
"I'm so getting you back for this, Kris."
Hoo boy.
---
Toriel lied in bed and stared blankly at the ceiling. Those big fluffy ears of hers had heard everything.
She didn't want to have the mental image of her son and the troublesome student boinking (that was what they called it now, right?) right next door on her mind all night.
'Asriel was never like this.' She thought. 'Well except for that one time with that Catty girl. And that alligator girl. Come to think of it...'
Her mouth bent into a frown. 'Wait, IS Asriel like this?'
With no further sounds coming from the boys' room, Toriel forced her eyes shut. Hopefully she wouldn't have to think about this tomorrow.