The owl from Cassian arrived at Hermione's Ministry desk the next morning, just as she was taking her first sip of tea. It was a large, stern-looking eagle-owl that dropped the scroll with a definitive thwack before fixing her with a disapproving glare and swooping out without waiting for a treat.
Unrolling the parchment, she expected a few sparse lines of data. What she found was… breathtaking.
It wasn't a report. It was a symphony of information. Intricate, hand-drawn schematics of the Vault's energy field pulsed with subtle colour-changing charms. Arithmantic equations she'd never seen before spiraled in the margins, calculating magical density and decay rates. His observations were meticulous, noting everything from the minute fluctuations in the hum's frequency to the way the ambient dust in the chamber seemed to avoid settling on the Vault's surface. It was the work of a true, obsessive genius.
And it completely undermined her entire morning's work.
She had compiled a list of seventeen possible historical parallels from Magical Meccas of the Ancient World and Pre-Gringotts Security Systems. After reading Cassian's scroll, twelve of them seemed laughably simplistic. He'd already theorized and disproven three of her remaining five with cold, hard data.
A fresh wave of frustration, hot and prickly, washed over her. It wasn't just that he was brilliant. It was that he was brilliant in a way that made her own methods feel slow and plodding. He was a scalpel; she was a sledgehammer.
She spent the afternoon cross-referencing his findings, her pride forcing her to find at least one hole in his logic. She found a minor inconsistency in his arithmantic modelling of the energy dispersal. It was a tiny thing, a rounding error, really. But it was something.
She penned a short, professional note. "Mr. Thorne, Thank you for the data. I have noted a potential anomaly in your third-phase arithmantic calculation. Please see the attached. - H. Granger."
His reply came within the hour, delivered by a tiny, speedy little brown owl that nearly crashed into her inkpot.
The scroll contained only two words, written in the same sharp, elegant script.
"Prove it."
---
"He's trying to get under your skin," Ginny said that evening, expertly Summoning a bottle of butterbeer from across the cozy living room of Grimmauld Place. Harry had insisted on a "team dinner," which mostly involved takeaway Chinese and dissecting Hermione's new professional nightmare.
"He's succeeding," Hermione grumbled, stabbing a dumpling with her chopstick. "He's arrogant, dismissive, and he acts like practical experience is the only form of knowledge that has any value."
"Sounds familiar," Harry said with a wry smile, dodging the napkin Hermione threw at him. "Hey, I'm just saying! Remember first year? 'There are more important things than friendship and bravery? Books! And cleverness!'"
"That's not fair," Hermione said, but a small smile tugged at her lips. "I was eleven. And I've learned since then."
"Have you?" Ginny asked gently, taking a sip of her drink. "Sounds to me like you've met your mirror image. A brilliant, stubborn know-it-all who doesn't play well with others."
The observation landed with the force of a Bludger. Hermione stared at her. "I am nothing like him."
"Aren't you?" Harry leaned forward, his expression turning serious. "Look, Hermione, you're the smartest witch I know. But you like to be right. You like to have the answers. So does he. You've just clashed with someone who operates at the same speed. You're used to being the fastest broom in the shed. Now there's another one."
It was an uncomfortable truth. She was used to being the undisputed expert in the room. With Cassian, she was constantly playing catch-up, and he seemed to relish her struggle.
"So what do I do?" she asked, feeling defeated.
"You do what we always do," Harry said with a shrug. "You prove him wrong. But maybe… you do it his way first. Show him you can play his game. Then, when you've earned a bit of respect, you hit him with the books."
It was sound advice. Annoyingly sound.
Later, as she was helping clear the plates, Harry nudged her. "Ron and Padma are coming by for a drink tomorrow night. You should come. Take your mind off Thorne."
The thought was appealing. The easy, familiar comfort of Ron's presence, without any of the old romantic tension, was a balm she needed.
"I'd like that," she said.
---
The next day, Hermione went straight to the Vault, armed not with books, but with her own set of enhanced diagnostic spells and a state-of-the-art magical resonance imager she had to sign six forms to borrow from the Department of Mysteries' auxiliary closet.
Cassian was already there, of course. He didn't speak as she entered, just watched her with that same infuriatingly appraising look as she began setting up her equipment.
For an hour, they worked in silence, a tense, unspoken competition filling the cold air. He would cast a complex, non-verbal charm she didn't recognize, and the Vault would pulse faintly. She would then run a scan with her imager, capturing the resulting energy signature.
Finally, she saw it. A tiny, almost imperceptible lag in the Vault's response to one of his spells. It was the proof she needed for her arithmantic theory.
"There," she said, her voice echoing in the quiet. She pointed to the graph scrolling out of her imager. "The resonant frequency spike is point-zero-zero-three seconds behind your charm's impact. Your model assumed simultaneity. That's your error."
Cassian walked over, his eyes scanning the graph. He studied it for a long, silent minute. She held her breath, waiting for the dismissal, the argument.
He looked from the graph to the Vault, then back to her. A slow, reluctant nod. "Hmph."
It wasn't an apology. It wasn't praise. It was a grunt of acknowledgment. But from Cassian Thorne, it felt like a standing ovation.
"Your equipment is adequate," he conceded, turning back to his work.
Hermione felt a surge of triumph so potent she had to bite the inside of her cheek to stop from smiling. She had done it. She had played his game and won a point.
"And my analysis?" she pushed, unable to help herself.
He glanced over his shoulder, and for the briefest second, she thought she saw a flicker of something that wasn't annoyance in his eyes. It might have been respect.
"Your analysis," he said, his tone dry, "is also adequate."
It was the best she was going to get. And as she packed up her things to leave for the evening, heading towards the comfort of friends and butterbeer, she realized that for the first time, the thought of returning to the cold, humming chamber tomorrow didn't fill her with dread. It filled her with a sharp, competitive anticipation.