Something they don't mention much in the anime/manga series as far as I know, but Mion and Shion, after the accidental tattoo/identity-switch, more or less spent the entire rest of their lives pretending to be each other. Aspects of their twin's personality would've rubbed off on both of their psyches, which you kinda can pick up on in the anime and manga. Shion's feminine act is basically how Mion thinks beneath all the layers of toughness, and once things get serious, Shion drops her dainty little act and will WRECK a bitch. Also during the Eye-Opening arc and throughout most of the sister's private interactions, you can generally see that Shion views Mion as having the weaker, shyer personality (and Mion lowkey acknowledges this), despite Mion being allegedly the older and tougher twin.

June 4th, 2020

When Mion and Shion were very young, it was quietly understood that Shion was the twin that their relatives did not like. So Mion was a good big sister and shared her clothes and her name as often as may be, and they managed.

But together, alone, their relationship was a little different. Mion was competitive, always wanting to do this, better, more, faster, and Shion was her tentative tag-along. Mion would climb trees like a monkey and scramble through mountain ravines like a goat, while Shion waited anxiously beneath or at the top of the ridge, ready to sound an alarm if Mion slipped and fell.

And then, that fateful day came. Mion was no longer Mion. Shion was no longer Shion. Oh, they could still switch, but it wasn't the same. People had a way to tell them apart now, a way that throbbed and pulsed painfully along Shion's back.

So Shion that was Mion took Mion that was Shion to a tree, and pointed to it. "We need to teach you to climb." she said. Shion that was Mion clutched her burning back and shook her head tearfully. She was only a child.

"They're going to be suspicious if you don't act like me." Mion that had become Shion pointed out. "C'mon. Its easy. I'll show you."

And she did, she showed Shion-that-was-now-Mion where to put her feet, how to scramble up the tree like a monkey and punch the air with a giddy shriek when she was done. And Shion-that-was-now-Mion clung to the thick branch and offered Mion-that-was-now-Shion a tentative smile, and said "You know, this isn't actually that bad. But if I've got to climb, you've got to stay on the ground."

And Mion-that-was-now-Shion frowned a little. It chafed her, to stand on the ground and wring her hands as Shion-that-was-now-Mion climbed soaring trees and scrambled through interesting ravines with increasing confidence, leaving her behind. How had Shion liked this?

The twins became known for pushing the boundaries, going farther and farther afield on the Sonozaki's expansive property as they sought to find a play-place where they could resume their proper roles, where Shion-that-was-now-Mion could stay on the ground like she wanted, and Mion-that-was-now-Shion could climb to her heart's content. And they found a few, but parents or Kasai would always find them to call them back, and what someone saw once, they could come up quietly to watch unseen again, and the twins had to move. Neither knew what would happen if their family found out about the switch, but neither Mion nor Shion thought it would be good.

But they found them, sometimes, and that was where the trouble, if it could be called that, started. Mion-that-was-now-Shion ran for the tree with a laugh and started up –and Shion-that-was-now-Mion was right behind her, clambering to the top. Silently, with flashes of teal eyes and clenched jaws and aggressive shifting of their small bodies, they challenged each other to climb higher, faster, first.

And Shion-that-was-now-Mion made it first with a laugh and a giddy grin, and Mion-that-was-now-Shion looked up at her from one limb down with a frown.

"How come you went first?" she asked. Shion-that-was-now-Mion tilted her head.

"I'm you now, aren't I?" she asked, kicking her heels gently as Mion-that-was-now-Shion came to join her on the top branch. "I've got to act like you."

"Well, yeah, but not…" Mion-that-was-now-Shion trailed off uncertainly, before she began again. "We're alone. You don't need to pretend."

Shion-that-was-now-Mion's grin softened, and she scratched her cheek a little. "Sorry…I guess its just become a habit."

"I guess." Mion-that-was-now-Shion did not understand why she didn't like that. Was she becoming like Shion too, now? It had become much easier to duck her head rather than flash defiance at someone when they scolded her lately…

A small twist of fear ignited in her heart. She was Mion! Shion was Shion! She shouldn't be acting like Shion, and Shion shouldn't be acting like her! It wasn't fair! That wasn't how this was supposed to work!

She was a good big sister now, and shifted uncomfortably on the branch. Hadn't she always disliked Shion's meekness? This was a good thing. Shion had broken out of her shell more permanently by having to pretend to be Mion for so long. That was good. And plus, didn't she have someone to truly play with now…?

Mion-that-was-now-Shion grinned and nudged her sister. "Let's try another tree! I bet I can beat you this time for sure!"

"You're on!"

The twins became known as rabble-rousers, little hellions always darting off to play and compete against each other on the estate. Mion-that-was-now-Shion was careful not to win too often in their family's sight, since that did not help her situation amongst them, but when she and her sister were off alone, well…

No holds were barred.

And that, perhaps, more than anything, was why Mion-that-was-now-Shion slyly teased Shion-that-was-now-Mion about this boy, Keiichi, that she liked, saying that they should have a competition to see who could seduce him first. After all, Shion's façade was the inner Mion that he liked, and Mion's façade was the inner Shion that still paced uncomfortably inside its restrictions.

She wanted to see if he would notice, and more than anything, it was a chance to tease her younger sister and start up another competition.

11.28 AM, USA Central Time