Kali.
I swore an oath to protect everyone with my sword.
Basic Information:
Name: Kali Aldebrand
Age: 25 years old
Height: 165 cm
Birthday: 14th May
Vision: Pyro
Weapon: Claymore
Affiliation: Knights of Favonius
Constellation: Ignis Gladius
FC: Snow White from SINoALICE
VC: Reina Ueda
More about Kali:
Despite her cold exterior, her personality is quite the opposite. Kind words and those of encouragement will be aimed at her allies or whoever she feels deserve them. It's not too unusual to see her smiling. Righteous and responsible at work, she always enjoys doing fun things. Even if she doesn't quite have the chance of doing so. She is respected amongst the Knights and the inhabitants of Mondstadt, to the point that some seem intimidated by her. Kali's strength in battle is worth of the position she is in, the Cavalry Vice-Captain, and she is very reliable. When the situation turns tougher, she will ask other Knights or any civilian to step back so she can take care of it herself without the risk of hurting anyone.
STORY
Kali used to live in a village away from Mondstadt with her parents and her little sister, Lily. She had a happy life, and she adored her family. That is - until her village was attacked by a powerful enemy that she would later learn was an Abyss Herald. Why did he attack her village?
The worst part wasn't her village being destroyed, but her sister dying in her arms. Covered in her sister's blood, Kali charged at the enemy, tears in her eyes and knowing she was no rival to him. She knew it, but... What else could she do?
Kali was defeated without a problem, yet her life was forgiven. Why? Another question without an answer.
She was 15 when that happened. But Kali was lucky a woman took her in and trained her to wield a sword. Kali didn't exactly seek for revenge, but to be able to protect people when evil came. She swore to her sister's grave that she wouldn't let anyone die with her being present.
Hestia was the name of the woman, her hair was of a fiery red that was quite skilled in combat. Even her was amazed at how quickly Kali learned and how natural her reactions and moves in battle were.
When her training was completed, Kali travelled to Mondstadt to enlist in the Knights of Favonius and in a relatively quick time she ascended throughout the ranks until becoming what she is today: Cavalry Vice-Captain.
Herut.
Herut was the name where Kali used to live, a tribe that was absolutely destroyed by the Abyss Order when she was a kid and her being the only remaining survivor. But what is this tribe relation with the Abyss?
They were descendants of an ancient civilization and wished to preserve their own culture, not wanting to be wiped out like Khaenri'ah. Even if they met a similar fate in the end. In order to survive, they allied themselves with the Abyss Order to search for protection.
It all changed when the Order asked for something specific. And that was what they called "the key". A powerful artifact that the Herut couldn't afford to give out. Even with the threats, they didn't change their mind. And since the Abyss Order couldn't get what they wanted peacefully, they decided to take it using the force.
Not even kids were saved from being slaughtered, the only survivor being a young girl with silver hair, that fought a herald even when the difference in strength was clear.
As to why they let her alive... It's because "the key" got implemented in her body without her being aware of it. But of course, later in the future, they would return to her and make good use of that weapon, when the time came.