Why had Robin been carrying that silver metal case all along?
Obviously, there was something very precious inside.
That very precious thing was a Devil Fruit.
Kahn had taken that Devil Fruit as spoils a few years ago.
— the Paramecia Fortress-Fortress Fruit.
The Fortress-Fortress Fruit was also known as the Castle-Castle Fruit.
As the name suggests, whoever eats it will have the inside of their body turn into a castle.
The user can also shrink any object, themselves included, and store it inside the body.
Even without going inside, the user's body in the outside world becomes a moving tank.
In the original story, this Devil Fruit is also Capone Bege's ability.
In fact, had Kahn's parents in this life not been killed, he would have given this Devil Fruit to Capone Bege.
But after their deaths, Kahn came to regard Capone Bege as merely a tool.
Therefore, after obtaining the Paramecia Fortress-Fortress Fruit, he did not bestow it on Capone Bege.
He planned for the Grand Line to recruit a new maid and train her into a mobile warehouse.
Now the time had come to create that mobile warehouse.
Kahn took the Devil Fruit from Robin, looked at Sonia and Mary, and said, "This fruit is one of the sea's secret treasures. Eat it, and you gain a special ability."
"This Devil Fruit's power lets you become a castle. You gain castle-grade defense, and your body holds a space as vast as a castle."
"With it, we can easily take all the gold here."
"But only one person can eat a Devil Fruit. Sonia, Mary—who wants to eat it?"
Hearing this, Sonia and Mary—and their elder sister Hancock—came over.
Studying the fruit in Kahn's hand, Mary asked in wonder, "Sister Robin has been carrying a Devil Fruit?"
Sonia was curious too. "Did Sister Robin eat some fruit to gain that special ability?"
"If you eat this Devil Fruit, you'll have castle-like defense?"
Hancock echoed Kahn's words, then suddenly said to Mary, "Mary, weren't you planning to major in Armament Haki? Then you should be the one to eat this Devil Fruit."
"Eh—me? You want me to eat it?"
Mary pointed to herself, then glanced instinctively at her second sister.
"I think so too."
With their youngest sister only nine this year, both Hancock and Sonia worried about her.
Especially since her curiosity had dragged her sisters into trouble, Hancock had long felt deeply guilty.
So now, any good thing should go to their youngest.
And Mary, being the youngest, was first in line.
Reading the two sisters' hearts, Kahn handed the Paramecia Fortress-Fortress Fruit directly to Mary and smiled. "In that case, Mary, I'll trouble you to be my mobile storehouse from now on."
"...I won't let you down, Master."
Glancing at Kahn's and Robin's smiles and at her two sisters, Mary felt a warm surge inside.
As a Kuja warrior, now free of her shadows, she didn't dither. She set down the gold bar, took the fruit from Kahn, and spoke firmly.
Then she bit into the fruit.
And the cool Mary instantly turned a blotchy purple-blue.
As she gagged and was about to spit out the pulp, Kahn reminded her, "Don't spit it out. One bite is enough. All Devil Fruits taste like that."
"...Gulp!"
Following his words, Mary forced it down. Then, looking at the remaining pulp, she gasped, "Hah... hah... this fruit is way too nasty!"
"Sour?"
Seeing her theatrics, Hancock and Sonia asked curiously.
"Not sour—just a mix of indescribable tastes."
As she recalled it, her face went purple-blue again; she felt awful.
Kahn and Robin, as the adults, took no pleasure in it.
Kahn steered the topic. "Mary, do you feel any change in your body?"
At that, the Boa sisters' attention shifted at once.
Mary blinked, then brightened. "I feel it—my body's changed a lot... Master, sisters, look!"
Lacking the words, she couldn't describe it.
She picked up a gold brick from the ground, reduced it to the size of a grain of rice, and put it into her body.
Put in, not absorbed.
A thumb-sized door opened on Mary's body.
Beyond it was not flesh and bone but a brightly lit castle interior.
The inside of Mary's body had become a castle, complete with its own lighting.
Such an impossible scene was only possible thanks to a Devil Fruit.
"Good. From now on Mary is my chief treasurer."
Watching her toss rice-sized gold through the little door into herself, Kahn smiled in satisfaction.
"Her insides turned into architecture—amazing!"
Sonia gaped, then asked hopefully, "Mary, can you put me inside too?"
"Mm-hm... I can."
Mary nodded hard and placed a hand on Sonia.
The next second a translucent white veil appeared out of thin air.
Wrapped in the veil, Sonia shrank before their eyes and flew through the open door into Mary.
"Master, Sis—want to go in and take a look?"
After sending Sonia in, Mary asked Kahn.
"Let's go have a look."
Kahn nodded.
Robin and Hancock did the same.
And the three of them shrank and entered Mary's castle.