Karin had a tragic life so far.
Seeing her mother getting bitten daily, and dying because of it.
And suffering through the same pain of getting bitten since the age of 4.
So, when she saw her 'teammates' dying by getting their heads cut off, she couldn't help but feel hopeful. That she might be able to escape.
And, an escape came.
Someone was hugging her from behind, and whispered, "Do you want to come with me? I will help you get your revenge."
She could feel that the chakra behind was ominous. But, she felt like her life outside Kusa wouldn't be that bad.
And revenge? She can take it?
"Yeah..."
She agreed.
And then, she lost consciousness as pain seered through her neck.
Ichi's Shadow Clone has given Karin her own cursed seal. To make her more attuned to her spiritual energy.
She then took Karin to her ROOT base where she experiments with the symbiote.
After getting her DNA sample and acquiring her abilities, she started her experiment. Failure on this test subject is allowed.
First, she implanted one of her egg with into Karin's womb with an operation, and carefully inserted the symbiote sperm for it to go to her egg instead of consuming Karin's body.
Then, she took off Karin's clothes and made her wear a bodysuit made from her symbiote's thinest, most basic cell.
It didn't really have any adaptive abilities, and was a rather chakra draining parasite which needed to be fed on the brain's chemicals.
But, it would help in nurturing the symbiote baby's formation, and the baby will then take over this bodysuit.
After a while, Ichi's egg met with her symbiote mimicking a sperm, and fertilized successfully, starting to divide within Karin.
No, fertilization would be the wrong term. But, it was creating a symbiote baby within Karin.
The Curse Mark in Karin spread all over her body, covering her into Shiori's chakra.
Normally, the bodysuit was more of a burden than anything for normal people. It just increased the physical capabilities a lot.
But, when combined with the Curse Mark, it started taking root in Karin's body and mind.
Ichi watched in anticipation as Karin's body convulsed in pain and the symbiote baby started spreading all over Karin's body following along the path of the cursed Mark.
It absorbed Karin's DNA and Chakra, and perhaps realising that she is some sort of her mother and has relation to Ichi, instead of eating her outright, started combining with her body.
The baby symbiote took over Karin's body completely, and unlike Ichi, whose symbiote is a different entity from her, they became one.
Ichi, for the first time, felt another entity in her mind. A hive mind, with her being in the centre of it.
The baby symbiote took over Karin's chakra, and memories, and because it didn't have any sense of self and somehow recognised Karin as important, they became one. The same way Carnage and Cletus would have been one entity.
Ichi watched in fascination as Karin, now more of a red symbiote with golden strands, groggily woke up.
Now, it's time to determine whether if she is Karin or symbiote to determine if the experiment was fully successful or partially.
---------------------------
Karin woke up groaning. Only to find herself in some sort of lab, having some sort of red and golden goo all over her.
She then looked at the girl in a purple bodysuit looking over her with interest. Karin instinctively felt like she is her creator.
Creator?
"W-what have you done to me!?"
"No need to worry, Karin. I have given you the power to get your revenge on your oppressors. Just think of your appearance, and you will change back."
Karin thought about it, and she looked like she was back to normal.
"Why don't you think about your scars disappearing? I have healed you."
"Huh?"
She thought about it, and her scars really disappeared!
"Now, let's have a test.
Who are you?"
"Karin."
"Do you feel any different?"
"I mean, yeah? You have turned me into this?"
"It's a powerful Kekkai Genkai that I have granted you."
"Powerful... yes... I feel really powerful..."
She didn't have any sort of ninja training. She couldn't even control her before.
But now, she felt like she had complete control over it.
"Do you hear me?"
Karin was surprised to hear the girl's voice in her head.
But, it felt strangely comforting. Like talking to her mother.
"Yes..."
She replied back in her mind.
"Alright. From now on, you will be my assistant. I will train you from now on, and when you are ready, you will get your revenge on Kusa.
And then, you will be an official member of ROOT.
We have no name, no face. We only work for the growth of Konoha!
Your codename will be Red. But, you have to first prove yourself capable, that you are worth of the codename!"
"I will do it! I will do anything you ask me to!"
"Then you can call me Ichi, my codename. And my only name."
"Yes Ichi-sama."
"Then, follow me. I will take you to our leader, Danzo-sama. He represents the ROOT."
Karin followed Ichi, but her face had a crazy expression, full of devotion and un-hingedness.