Ok so
Since this is really long, I'll have to do this in parts. I just finished writing this one and it took me 3 hours as I'm following the manga to fact check and also bc I'm slow at this kind of things. If there are questions, feel free to ask
Relevant stuff that I mention in this part without me explaining it before:
>Schicksal: An organization founded long ago by the Apocalypse, Kaslana and Schariac families to fight against the Honkai. Its main fighting force is comprised of Valkyries, girls with an high resistance to the Honkai, allowing them to be infused with it to strenghten their combat capabilities.
>Otto Apocalypse: The current overseer of Schicksal. He's been around for about 500 years at the time of the game's events. He managed to survive this long by using replacement bodies. He's obsessed by Kallen, and his end goal is to find a way to revive her.
Main Story
Nagazora Arc
Kiana, Mei and Bronya find themselves in the middle of an Honkai eruption in Nagazora. To save Mei, Kiana gets heavily injured. This causes the 3rd Herrshcer to awaken. While Himeko, who got there on the Hyperion, was fighting her, Kiana regains her senses and tears off Herrscher Mei's wings, defeating her. The two girls get rescued and brought to St. Freya. Bronya was later found on the ship as well.
Theresa convinces Kiana to enroll at St. Freya's school for Valkyries, after agreeing to tell her the whereabouts of her father if she manages to become an A-rank Valkyrie. Bronya and Mei had enrolled too, with the latter having a micro-explosive implanted next to her heart in case the Herrscher were to awaken again. The girls get to stay at Himeko's home.
First Exams
A month after enrolling, the girls have to take their first exam. Bronya and Mei get the 2nd and 3rd highest scores, respectively, with Best girl Fu Hua, who was previously shown in class, in the 1st place. Kiana was also first... but starting from the bottom. She had failed all the written exams. She now has to take a remedial exam, and Bronya offers to help her study, giving her a VR comm link, using it to access Schicksal's database on the history of the Honkai. With it, she starts showing her relevant historical events in the fight against the Honkai, but after reaching the 14th century, Kiana finds in the simulation a reproduction of Theresa's cross, the Oath of Judah. She touches it, and after recognizing her as part of the house of Kaslana, hidden memory files gets decripted and she ends up falling in Kallen's hands, in an extremely realistic simulation of the last days of her life. Kallen was to be executed by hanging, due to having betrayed Schicksal by stealing an extremely imprtant artifact and escaping to the far east (where she met Yae Sakura). Kiana tries to fight those who had captured Kallen, but gets defeated and both get imprisoned. "Kiana's mom" gets her out of prison, and since Kiana is a big ignorant idiot shows her who Kallen was and why was she executed. After listening to her story, Kiana decides to save her, even if only in the simulated world. Bronya agrees to help, and comes up with a plan.
The day of the execution, after the explosion Kiana starts fighting the hangman, but instead of fighting back he shows her an escape route and lets her take Kallen. But there was someone else who also devised a plan to save her. Otto Apocalypse, who at the time had yet to become Schicksal's overseer, let free some previously captured Honkai beasts, so that he and Kallen could escape in the midst of chaos. But Kallen refuses to go with him, and instead starts fighting the beasts. Meanwhile, Bronya had found an hidden record in Schicksal's archives, showing how Kallen didn't die by hanging, but was instead killed by an honkai beast on that same day.
After that, Otto found Bronya, and she noticed he wasn't a VR generated character. Due to their willingness to save Kallen, he lets Bronya give Kiana proper equipment to fight the beasts. After defeating all of them, Kallen reveals to Kiana that she knew that she was only a VR copy of the real Kallen, and that she lived those two days over and over, with them all ending with her death no matter what she did. She then gives Kiana a message for Yae, who the real Kallen sealed away 500 years ago, in case she were to ever meet her.
