This takes place shortly after Sholmès' route good ending (so Cardia's still not cured of her poison), but also assumes that Cardia lived the Lupin's Gang ~Another Story~ (somehow) and still became friends with Shirley. Because mixing routes is funny and it doesn't hurt.

The sunlight made it sparkle beautifully. This thin long golden hairpin made Cardia gulping with envy.

Normally, when she went on shopping along with Shirley, she did not desire much. She cherished being and spending time with her friend, having fun with her and making memories, much more than any gift or material matter she could get.
Surprised by her unexpected philosophy, people often complimented her that it was a splendid way of thinking -she reminded the cleverer ones of a modern Epicurus, accepting joyfully the gifts her friends and life offered her, but not wishing much more herself. She had no idea of greed or superficiality.

Yet, on this afternoon she was fully enjoying with Shirley, her eyes got stuck, for the first time, on this cute hairpin in the jewellery store. It had wonderful golden reflects and gorgeous blue and purple glass pearls attached to it, like they were floating in the air. An obvious oriental inspiration. They would oscillate and follow her body and hair movements so gracefully…

Simply exposed in the jewellery, she stared at it without blinking for an unhealthy amount of time.
She… wanted it.

"Oh my, Cardia!" Shirley exclaimed as she approached her friend, her hands full of sweets she just bought. "You really need to taste the red ones! They're absolutely deli-hum? What are you looking at?"

As innocent as a child timidly asking for a toy, she pointed at the accessory that shined through the pane. As if the sparkles the jewel projected contaminated her, Shirley's eyes suddenly started glowing with admiration as well.

"Woaw! What a beautiful hairpin!" she happily shouted, almost putting her small hands on the pane in excitement.
"…Yes… It is."

Shirley looked at Cardia's shy smile and soothe irises by the corner of her eyes. It did not take her long to deduce what the young woman was thinking. Actually, less than a second.

"Cardia… Would you like to have it?
-Oh, well, I do…" she admitted. "But it'd be irresponsible to spend so much money on such a thing, so I guess I'll restrain myself this time!" she added with an awkward smile.
"…'This time'…" Shirley repeated with a pout and a silly voice. "You always say that whenever we go shopping! It's alright to desire something, Cardia! You always restrain yourself when your lover throws his money out at any bookshop he sees!"
"It isn't the same… That's his money and Sholmès needs them for work: it is his job to gather as much information on anything as possible, and I need to learn, too. One time he could deduce what kind of cigarettes the culprit smoked by the scent of the ashes!"

She said that with an admirative tone as she remembered with how much easiness he could tell on a person by simple details. But the only answer from Shirley she encountered was a perplex face.
"What a weirdo."

Cardia instinctively blushed at the mention, not so much of shame than just remembering that Sholmès definitively was an eccentric. But she eventually became accustomed to it and barely noticed it anymore.
Though it was still a bit difficult to endure his sudden urges to play German classics on his violin or do odd chemistry experiences in the middle of the night -even when she would go to ask him to be quieter, she would not find him sitting on the sofa but on a strange disposition of cushions, just like an Indian or Arabian prince, just because he found it more comfortable.

"He's trying to improve himself!"
It was the only protest she could express which was true: he was not playing loud German music anymore, but soft French ballads. It was better than nothing.

"Anyway, even if I had the money for the hairpin, it would be use-"
"Ah! If you can't buy it, I will!" declared proudly Shirley. "Just wait and see!"
"Ah- Shirley!"

The tiny lady grasped her wrist with as much strength as expected from a mafioso's daughter and dragged her against her will in the store.

Soon, whether she wanted it or not, she found herself being offered the golden hairpin. 'A gift for my non-official big sister', as Shirley said.

The day afterward, in the early morning, its fairy glow on her dressing table still proved that she did not dream this unnecessary purchase. She sighed at the sight of it. Shirley was so very happy to offer it to her… Yet…
But, she thought to herself, she'll be disappointed if it is left unused… I should at least try.

As every morning, she gently brushed her long, lightly curly hair, a bit bullied by a night of rest on a pillow. Once it was perfectly untangled, the morning sunlight made its hazelnut-alike hue gleam beautifully.
Cardia arranged it in a high ponytail, then picked up the hairpin. She inserted it carefully into the ribbon that tied her hair up… slowly put her hands down. Looked herself in the mirror.

For the first time in the morning, a smile brighter than the clear sky outside appeared on her face. The hairpin's dangling jewels waved and swung along her movements, just as she expected…

While she always thought of herself as an average girl, for once, in the mirror's reflection… She found herself worthy to be called 'princess', as Lupin's Gang was determined to call her.
She was almost about to burst out in happy laughter when a sharp noise interrupted her- a boiling sound.

She instantly pulled out the hairpin out, only to see its extremity burned down by her poison.
Just as expected, it easily touched her scalp and melt down just like Sophie's wax doll in the sun in Sophie's Misfortunes. A book she did not read at the Detective's Agency -one she read when she was still in her mansion in Wales. A book who told her to well behave, to be a good girl whatever happens, as her father asked her. Otherwise, she would as unhappy as Sophie was.

And, undeniably, the disappointment and sadness in her eyes as golden hot tears dripped to the ground was the same as the fictional little girl.

I won't be able to use it after all… Shirley and I were so excited about it… It's just… So unfair…
She consoled herself thinking she could perhaps attach it to her bag or her pocket -when, suddenly she heard a door shut.

It was nothing brutal, but enough to surprise her on the instant. She instantly turned… only to see Sholmès multitasking on the table. Though she did not know if putting on a necktie with one hand, brushing his long purple untied hair with the other, chewing on some food while reading reports was useful and healthy multitasking…

They brightly greeted each other like every morning -but for once, she could not get her eyes off his hair while he tied it up in his usual high ponytail.
Mmh…

He eventually noticed her insistent stare.
"Is something wrong?"

"The crime scene is just there, behind the door."

Cardia nodded with energy and carefully rushed to the room -though it was really difficult for her to investigate corpses, especially in violent murders, she wanted to be as useful as Sholmès and Watson and fought her repulsion by going first, so her two fellows would not advert her eyes.

Sholmès slowly followed her, calmly walking as his usual self -Watson, on the other hand, stayed still, desperately staring at his best friend.

"…Is something wrong, dear Watson?" finally asked Sholmès with a mildly annoyed tone. "You've been staring at me since this morning."
"Well, it's quite difficult not to stare at you when you wear such a thing on a crime scene, Holmes."

The golden hairpin glimmered mischievously in the detective's purple hair.

BONUS:

He needed to crack the code.

It was in his instinct. As silly as a child playing: pick up a toy (a code, a dilemma, an enigma, a riddle, whatever that was, as long as it had mystery), mercilessly use it to the core until it breaks, throw it, pick up a new one. A way of keeping him entertained -even though, to his great despair, it was almost always too easy.

However, he learned to know that the codes that seemed the easiest and the simplest at first sight were the ones to keep hidden their secrets the longer, and furthermore, the hardest to crack. And the pinnacle of that was in front of him -incarnated in the young woman he fell in love with and who was now serenely pouring hot steaming tea in their cups.

He responded to it with a smile, while his gloved hands played with the hairpin she offered him not so long ago. A way to distract his body when his brain was fulminating with reflexions and thoughts. The blue and purple glass pearls were dancing as he was twisting the pin between his forefinger and thumb.

He noticed. How could he not? It was his job to notice, deduce, reason. Yet, the last two actions were not conclusive at all. Not. At. All. He could not get it at all.

He loved when the mystery resisted a bit to his will -just like a cat playing with a mouse before killing and eating it. Gathering the facts… Reconstructing what happened -what reasons, what alibi, what truth? But it lasted for too long. It was not amusing anymore. Just… frustrating. Terribly frustrating. And most of all… why did he care so much?!
The pressure of his fingers on the hairpin increased dramatically.

"Are you alright, Sholmès?" she suddenly asked in her soft voice, blinking lightly. "You're going to stay petrified, if you frown so much… You did not even touch your teacup."

That was it. He gave up, for the first time in his career. He could not bear not knowing the truth -even if it meant to dishonour his detective name.
"I was just wondering…" he began in his calm voice as he raised the teacup to his lips. "You really enjoyed wearing blue and purple lately, did you not? How come?"

She cutely inclined her head a bit, the newly-acquired blue and purple stripped ribbon closing her collar following her reaction as well. It was a new gift of Shirley, as a replacement for the hairpin she could not wear. She smiled, amused.
"Of course I enjoy wearing blue and purple, Sholmès. Because… these are our colours."

The hairpin instantly snapped in his powerful hand.

Sholmès could not even remember when was the last time he overreacted, and maybe, blushed -perhaps it even was in his childhood. But it did not matter in any way, since the amount of blood that rushed to his face and reddened his cheeks was far greater that any he ever experienced.
It felt like his flesh was straight burning in the fires of Hell -this girl definitely knew how to make him melt… in more way than one.

"Sholmès! You broke it! –D-Don't choke on your tea!"