That was a really fun prompt to write for wilhelmares on Tumblr!
I laughed the entire time because we all know Mari can be a total pranking silly ass ;)

"I won't lie to you…", Alya began and Marinette gasped.

"So you ate that last piece of cake!", she indignantly shouted, pointing at Alya as she straightened up and closed the fridge. Alya confusedly pulled her eyebrows together, slowly shaking her head.

"Uhm… No… That's not what I meant-"

Marinette threw her arms up to hide an amused smile, rushing past her girlfriend to flop down on the couch.

"Oh nnnooooo, I was sooooo looking forward to that after coming home from work today!", she whined, pressing her face into a pillow not to let Alya see her suspicious smirk. She could hear her girlfriend following her, then she felt the couch being pushed down as Alya sat down by her legs.

"No, I-… Mari, please listen."

The girl turned around and sat up, her mouth forming a trained 'o' as she put her hands on her cheeks in a mocking shocked expression.

"Oh no, wait, I know. Adrien came by and ate it and he knocked you out to get to it."

Alya rolled her eyes but before she could retort anything Mari already pulled her against her chest, patting on her head.

"Poor, poor, defenseless girl. I'll make sure to punish Adrien the next time I see him. Maybe giving him uncomfortable clothes to model next time will help."

"Maaaaauriiiii…", Alya whined but her girlfriend wouldn't let her have another word as she already pulled her up to eye level, squishing her cheeks together.

"Oh. My. God.", she narrowed her eyes and Alya already felt herself getting nervous at the intense stare, "It's-… It's you…"

Alya sighed and freed herself from Marinette's grip, pressing her lips together.

"Listen, I was gonna tell you-…"

"You double crossed me! Adrien was never here in the first place! You just set that up to confuse me! You ate that cake, am I right?!", she stood up and gasped loudly, her eyes wide, "… You lied to me!"

Alya groaned as she let herself fall back against the backrest, getting annoyed at her girlfriend's childish behavior.

"Would you please just listen to me?!", she begged, rubbing over her face, "I have to tell you something important now that we've moved in together, okay?!"

The cushions were pushed down on either side of her thighs as her girlfriend sat down on her lap, her knees resting on either side of her body. Alya looked up to Mari as she grinned down to her, nudging her nose against hers.

"Can I please tell you now?"

Marinette smugly shook her head, cupping Alya's face and propping herself up on her collarbones.

"Nope. I'm still talking. You know, I've known for a long time now.", her fingers brushed over the earrings on Alya's ears and her breath hitched, "And you don't have to worry about anything, Alya. Your secret is safe with me. After all, I'm the one who gave it to you."

Alya's eyes widened at her girlfriend's words and she struggled for a response as Mari already continued.

"… I ate that slice of cake.", she smirked.

Alya groaned and was about to throw her girlfriend off, pin her down on the couch and just yell out that she had a Miraculous and had been a superhero since tenth grade when she found the earrings in her bag at the very first Akuma attack but a kiss completely threw her off. Marinette's soft lips cut out any thought and Alya melted into it, her arms coming up to wrap themselves around her slim waist.

But before she could get to kissing her girlfriend properly, Mari already pulled away.

"Oh yeah and I know you're Ladybug."

Alya immediately had a coughing fit.