Izumi remembered watching her father getting killed in the Kyuubi attack 12 years ago. She awakened her Sharingan, but didn't really feel happy about it.
She then moved into the Uchiha clan compound with her mother, and started going to the academy with the boy who saved her from getting crushed by rubble.
Uchiha Itachi.
And somewhere along the way, she caught feelings for him. While he became an ANBU captain, she became a Chunin.
Everything was going great, and then suddenly, she dreamt about Orochimaru taking over her body, and killing Itachi.
At that time, she didn't realise that it was a dream, everything felt too real.
And when she woke up, she saw Itachi killing her mother in front of her. And as blood dropped from her eyes, she was knocked out.
When she woke up again, she was in the hospital. Itachi had wiped out the Uchiha clan, leaving only her and his little brother alive.
And no one knew the reason why. As far as Izumi could remember, there weren't even any issues going on.
Was he just crazy?
Did he did so to try to awaken the Mangekyo Sharingan of many people? Did he kept her alive because she awakened it?
Izumi has never questioned it, but she instinctively felt that it was because he wanted Sasuke's eyes after having him awaken Mangekyo Sharingan. And he kept her alive just in case Sasuke wasn't able to. Though the possibility of rejection from her eyes will be greater, there will still be a possibility that he could get Eternal Mangekyo Sharingan.
Izumi didn't have the clearance to know such things. She didn't even know that Mangekyo Sharingan exists. Yet, all this information was engraved in her mind, and she didn't even question them.
When she saw Sasuke waking up, she wanted to kill him just to ruin Itachi's plans.
Yet at the last moment, even before the ANBU knocked her out, she automatically dropped that thought for some reason.
Instead, she wanted to protect Sasuke suddenly.
She couldn't understand what was going on with her, and then, she got kidnapped by Orochimaru.
She was given a Cursed Mark, which she barely survived. Because of her Mangekyo Sharingan ability protecting her.
Tamashi-no-kami: the god of soul
Karada-no-kami: the god of body
In the three years that Orochimaru trained her, inciting her with power, she secretly realised the powers her eyes held.
Her left Mangekyo Sharingan, Tamashi-no-kami allowed her to manipulate her own and other's souls. Using it, she absorbed the small part of Orochimaru's soul that resided in her curse mark to obtain all his knowledge.
She realized some things.
First of all, Orochimaru actually died once to Itachi in less than 5 seconds.
Second, he was able to resurrect using the curse seal. And he had Edo Tensei.
She now knew that she could resurrect the whole Uchiha clan. She alone didn't have the confidence to be able to kill Itachi, as she couldn't even completely subdue Orochimaru after she allowed him to use his living corpse reincarnation on her, intending to completely absorb his soul.
While perfectly resurrecting the Uchiha clan would be impossible for now, even Edo Tenseis would be helpful.
But, strangely enough, Konoha has changed its policy of funerals. Konoha now burns the bodies fully, and then throw the ashes deep into the ocean.
After fighting with Orochimaru's spirit for a long time, they came to a truce. She would leave this body behind in favour of a superior body, of Karin Uzumaki.
Her Mangekyo Sharingan is connected to her soul rather than the organ Sharingan. She has made it so with her left Mangekyo Sharingan.
With her right Mangekyo Sharingan, Karada-no-kami, the god of body, she could freely manipulate her and other's bodies to take any form, and even heal. Using this, she helped Orochimaru in transplantation of Hashirama cells into her body perfectly, ensuring that her body doesn't die.
Likewise, she got the abilities of Suigetsu, Jugo, and Sage Mode.
Yet, she still didn't feel confident in taking on Itachi. Atleast alone.
She sneaked into Konoha rather easily with Orochimaru controlling her body, and decided to find the DNAs of the dead Uchiha clan for Edo Tensei.
Yet, the place where her house was, in the outskirts of the Uchiha clan compound, stood a big palace. The palace of the scammer girl that she remembered.
She was able to get some DNAs from the main houses of the Uchiha clan, but they weren't of her mother.
The village has been more thorough than expected, and on using Edo Tensei, the DNAs revealed that they belonged to pretty insignificant people in the Uchiha clan, who weren't even genins.
As she was going through the emotional turmoil, she saw the scammer girl returning from Academy with Naruto.
And she saw it. She has two souls. One soul was normal as the other souls she has seen.
But the other soul was different. She could feel that the soul was connected to a different realm.
When she first used Spirit Transformation clone to exit her body, she saw the spirit realm of the world.
Everyone's spirit is connected to each other. The Edo Tensei's spirits are connected by a string to a realm that screamed to her that it shouldn't be touched. It's the pure land. From which Edo Tenseis draw their unlimited refilling chakra.
Everyone followed this arrangement, even the tailed beasts, who had an additional connection with their jinchuriki.
Yet, Shiori didn't. One soul followed the world's ways, and the other was connected to a different realm altogether, yet it felt awfully similar to pure land.
Since then, Izumi has been investigating Shiori. She often possessed her friends like Sakura, and even the clan heads like Hiashi Hyuga. She didn't even try to possess Shiori, and Naruto because of the second soul and Kyuubi, and stayed away from Karin who could even see the colour of chakra. And she obviously stayed away from Hokage, and Yamanaka clan.
She got the information that Shiori has an seer-like ability.
Izumi, and Orochimaru theorized that her second soul is actually connected to a parallel world, and the seer ability is her seeing things from that parallel world in her dreams.
While Izumi didn't want to kill Shiori for building a palace on the land of the Uchiha clan, she also wanted to travel to this parallel world she was connected to.
Maybe Uchiha clan is alive in that world?
Sadly, she knows too little about souls. While she has gotten many ideas from Shiori's manga Jujutsu Kaisen, specifically from Mahito, she isn't actually sure if messing with Shiori's soul wouldn't mess up her second soul's connection.
Afterall, she lost the seer abilities after hitting her head.
That's why she wanted to first possess Karin, make her more compatible to be her vessel slowly, and keep an eye on Shiori closely.
But her barriers unexpectedly found something wrong with Karin. Because of Dark Release barrier, a barrier made by the Chimera on which Shiori is working on.
Kakashi has a particularly strong Mangekyo Sharingan. And Shiori and Karin don't leave the village often. And the one time they did, Kakashi was there.
To go unnoticed for long so she can research in peace, she will use the Chunin Exams for letting Orochimaru complete his goal of killing Third Hokage, and amidst the chaos, take over Karin, destroy the barriers of Shiori's palace using the brute force from One-tails, and even if she fails, she can just posses others people she have in mind and are already under her influence.
And, she has a way to use her right Mangekyo Sharingan to directly manipulate Shiori's body without touching her souls.
Well, for now, she wants to destroy Itachi's plans.
With Hashirama cells, and her Mangekyo Sharingan directly embedded into her soul, she doesn't need Eternal Mangekyo Sharingan. But that's not the case with Itachi.
She will not kill Sasuke. She doesn't want to for some reason.
But there is no reason she can't just pluck out his eyes for laying a trap for Itachi.
Thinking about it, she appeared before Team 7, and looked at Sasuke.