Castillian #1: Sinner's Formula

The engineering lounge was quiet again. Cozy. Private. It was as if there was no one else in the world but them.

Scarlet sat across from Mico, nervously twirling her pen as she tried to make sense of the triangle in front of her.

"Scarlet," Mico called in his usual low voice, placing a box of donuts on the table.

She looked up. "Oh. You really brought—"

"Choco glaze," he cut her off. "Your favorite. The one you suck on like it's your stress reliever."

"You're too much. Why does that sound kinda dirty?" She laughed softly, taking one. "But thanks. At least now, the cause of my cavities tastes good."

"Donuts aren't the only ones with cavities," he murmured, eyes never leaving hers.

Scarlet blinked. "What—?"

"Focus," he said with a smirk, opening his notebook and sliding it toward her. "Let's start with the sine rule."

At first, the study session was quiet.

Scarlet was really trying. Mico was… well, half-trying. Because while she was busy writing solutions, Mico would randomly reach over, guide her hand… and the closeness felt a little too intentional.

Like the warmth of his palm resting over the back of her hand didn't quite belong. Or how he adjusted his chair just to get a little closer.

Or that one look he gave her—definitely not the kind professors give. More like a God-I-wish-there-was-a-curtain-here kind of look.

Scarlet tried to stay composed.

"So… a-over-sine-a equals b-over-sine-b?" She asked, voice slightly trembling.

"Yeah," he answered, staring at her lips.

She swallowed. "Then we cross multiply?"

"No."

"W-what? Why?"

"You multiply this first," he said, leaning in, grabbing her pencil, and letting his fingers ghost over hers. "But before that…"

Scarlet raised an eyebrow. "Wait for what?"

Mico gently cupped her cheek.

"I got distracted," he whispered.

And just like that—

He kissed her.

Softly at first.

Then firmer. Deeper. Wetter.

Scarlet made a small sound in her throat, caught off guard. But she didn't pull away.

He moved his chair closer. Way closer. Like this wasn't tutoring anymore. More like a recap of how he kissed her the other night—but more deliberate.

His hand moved from her cheek to the back of her neck, pulling her in. His lips parted hers, slow and in control, like even now, he was guiding her.

And she responded. God, she responded.

Mico tasted like peppermint and control issues. And Scarlet? Was melting into the kiss.

When he finally pulled away for a second, her eyes barely opened. "M-Mico…"

"Hmm?" He murmured, lips brushing against hers, eyes half-lidded and dangerous.

"This doesn't feel like… tutoring."

"Oh, but it is," he smirked. "Lesson one: distractions are inevitable."

He kissed her again. This time, with tongue.

Scarlet gasped into the kiss, and he took full advantage—deeper, hotter. His tongue explored slowly, savoring her like the donut he brought earlier.

Every few seconds, he'd pull away, only to dive back in. Soft moans. Messy kisses. Whispered curses.

"Lesson two," he breathed against her lips, "when something feels this good…"

He kissed down her jaw.

"…you don't fight it."

His mouth found hers again, and this time, there were no pauses. Just saliva. Tongue. Breathy whimpers and low groans.

"Lesson three," he said between kisses, "I'll never get tired of this."

"Mico…" he whispered, fingers gripping his shirt.

He kissed her again. Harsher. More desperate.

"I'm not stopping," he growled, "unless you say so."

Scarlet stared at him, dazed, lips wet and swollen. "…go find the next triangle."

He chuckled darkly. "I found it," he said, lips brushing her neck.

She gasped.

Then she laughed.

Then he kissed her again.

And this time, they both knew—

No equation could ever explain this.

- 🏀 -

Scarlet never imagined that being Mico Cein Esguerra's secret girlfriend would feel like this.

At first, she thought it would be stressful. The kind where she'd have to hide from people, always be paranoid, always on edge.

But no.

Instead, she was drowning in... benefits.

Literal ones.

Academic, emotional, physical... and very unholy benefits.

It started with the assignments.

"What's your next topic in Math?" Mico asked while they walked together toward the library. He was wearing a hoodie and a cap pulled low—like a ninja in the night. Scarlet walked beside him, pretending to look like a normal girl who didn't have a boyfriend this hot.

"Uh, logarithmic functions?"

He nodded, then casually reached for her bag and pulled it from her shoulder. "I'll do your seatwork tonight. Don't worry about it."

Scarlet stopped in her tracks. "Wait. You're doing my seatwork?"

"You want to fail?" he asked, deadpan.

"Of course not!"

"Then shut up and send the file."

"...Yes, boyfriend."

He glanced at her. "Repeat that."

Scarlet smirked. "Mico Cein Esguerra is my boyfriend. Happy?"

"Very."

Next was the food.

Scarlet was sitting at the farthest corner of the cafeteria, kind of hidden, trying to study while fighting off hunger. Then suddenly, someone passed by and slid a paper bag onto her table.

"Chicken sandwich. Medium fries. Large iced tea. With three extra ketchup sachets because you hate running out."

Scarlet looked up.

Mico was already walking away, hands in his pockets like nothing happened.

Damn. Having a boyfriend who has a photographic memory of your cravings is dangerous.

She texted:

Scarlet:

Was that a secret move? Is this how ninja-boyfriends treat their girls?

Mico:

You're a secret girlfriend, not a secret priority.

Then came the pick-ups and drop-offs.

"Wait for me at the gate," Mico texted one night after she finished an org meeting. "Don't ride a trike."

When she got to the gate, he was already there, leaning against a matte black motorcycle, helmet in hand.

"Hop in."

"Wait. Do you actually know how to ride a motorcycle?"

Mico stared at her. "Scarlet."

"…Okay, fine. Sexy points earned."

"I heard that."

But the worst—or best—part?

The kisses.

Dear God.

The kisses.

Scarlet eventually realized that with Mico Cein Esguerra, every study session came with a bonus package: tongue-shoving, moan-inducing kisses.

Even in the library? Yes. In the lounge? Of course. At the fire exit of the Engineering building? Don't worry, apparently there's a CCTV blind spot.

"Focus," he'd whisper while guiding her hand in solving problems.

But while she stared at her notebook, she'd suddenly feel damp lips on her neck.

"Wait—Mico, this isn't part of the formula—"

"Quiet." Slurp. "You were tense."

"SHIT."

Then he'd kiss her.

Hot. Deep. Wet. With every damn intention of turning her into jelly.

Tongue first. Slurp. Then a nibble. Then a gasp. Then his mouth would suck on her bottom lip like she was his favorite candy.

There was even one session where she ended up sitting on his lap while he told her to read the solution out loud.

"Read it," he whispered, while his hand slid under her skirt. "Loud. Clear. No stuttering."

"I can't read this 'a'…"

"You mean 'A'? Capital A?"

"I hate you."

"You love me."

Then he'd kiss her again—this time with such precision, even her brain forgot how to divide.

And the dates?

Oh, they were never just dates.

They'd eat in quiet cafés, always at the corner seat. And under the table, she'd feel his hand on her knee, going higher.

"Someone might see—"

"No one's looking," he'd say, while casually slipping his fingers between her thighs.

No one's looking, my ass.

Mico even developed this dirty little habit of squeezing her ass whenever they were alone in elevators.

There was one time in the library elevator, going up…

"Ssssh," he said, while kissing her neck.

"There's CCTV."

"Dead cam. Felix checked it last week."

"WHAT?!"

Then he'd grope her—firm, possessive, hot.

"Anywhere, Scarlet," he'd whisper, lips brushing her ear. "As long as it's you."

And yet, despite all that—Mico was always chill. Nonchalant. Stoic. Mr. Icy.

Even after an intense make-out session, he'd go right back to studying. Like no soul-sucking had just happened in the corner.

Scarlet, on the other hand?

Drooling. Dignity? Gone. But happy.

Even if their relationship was still a secret, she was starting to think…

Maybe being Mico Cein Esguerra's secret girlfriend wasn't so bad after all.

Especially when every kiss felt like detention in heaven.