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Kallen Kaslana is one the major protagonists of the Honkai Impact 3rd and Guns Girl Z video game series, and a posthumous protagonist from the manhua adaptation.
Kallen is the ancient ancestor of Kiana Kaslana and the most famous Valkyrie to have ever lived. She eventually became Otto Apocalypse's obsession, thus causing him to genetically-engineer a clone named Theresa Apocalypse who has the DNA of Kallen and a Honkai beast named "Vishnu". She has romantic feelings for Yae Sakura after she found her unconscious in a river.
Kallen's battlesuits are the Ritual Imayoh (A-rank), Sündenjäger (A-rank), and Sixth Serenade (S-rank).
As a child, Kallen’s first appearance is when she witnesses Otto Apocalypse’s toy plane fly over her outdoor wall. Kallen grabs the toy and climbs over the wall, returning it to him. She is very impressed when she learns he made it. She offers Otto a chance to join her “quest” to ‘save the world and make it a better place'. The two became close friends from then on.
Still young, she and Otto run into danger. Sir Francis, Kallen’s father, saves them. He fights off hordes of Honkai beasts before being consumed by Shamash's flames. Later, at the funeral, Kallen cries the entire time. Otto leaves, and minutes later comes back, saying he can return Sir Francis to life by giving his own. Kallen slaps him, saying that her father gave up his life to save them, and that if Otto did this he would be throwing it all away for a lie. Otto apologizes, and Kallen says she won’t let anyone die for her.
The First Samsara
In the first Samsara- the current events- it was where Sakura first found Kallen injured by a river, holding the box. When the 12th Ruler possessed Sakura and made her a pseudo-Herrscher when she opened the purple box, Kallen was eventually forced to seal her loved one away with the Judah's Oath.
The Second Samsara
In the second Samsara, it still follows the current events but this time, Sakura was completely taken over by the 12th Ruler and fought with Kallen. Unfortunately, Kallen -despite her abilities- could not defeat her former lover and was killed by the Houkai-possessed Sakura as a result.
The Third Samsarae
In the third and last Samsara, Sakura successfully regained consciousness from the 12th Ruler's clutches and broke from its control. Kallen and Sakura fought together and they managed to defeat and seal the Ruler in the purple box and purify the Houkai energy that was plaguing the village for a long time. The two lovers are finally able to live a peaceful and happy life thereafter.
The Side Samsara and Soul Throne Raid
Kallen and Sakura were lying down on a meadow on a peaceful day, holding hands, until Kallen released her hand and told Sakura that she needed to leave. This prompted Sakura to search for her loved one in the village. However, the villagers were acting strangely, and Sakura decided to investigate it. Sakura then found the creature, Higyokumaru in a barrel- unaware that she was a Ruler- and the two sought out to find the former's memories. After a long period of fighting beasts and monsters on the way, Higyokumaru then regained a portion of her memories upon seeing a peculiar mask.
Higyokumaru then touched an odd sword, which made her transform into Jizo Mitama, which Sakura defeated. After reverting back to her old self, Higyokumaru fully remembered her past that she was the 12th Herrscher of the Old World and has killed many people. She apologized for the troubles she caused, which Sakura accepts. Higyokumaru then told Sakura that the Kallen from her memories was an illusion. Sakura apologized and said she knew that the real Kallen was no longer in the outside world. Even so, Sakura still wanted to find Kallen in the Stigmata dimension.
Unexpectedly, Kallen appeared in her Phantom Thief serenade and stole the Jizo Matama sword away and swiftly escaped. Sakura immediately recognized her voice and went to chase her along with Higyokumaru. When the two found Kallen again, the latter apologized and chained Sakura with her Judah's Oath in order to stop her. Kallen, who lost her memories, said that she had to take away the sword because it was too dangerous. Sakura then reached out her hand and touched Kallen's face and told her that she was not there for the sword, but she was there for Kallen. This made the latter's memories come back, but the sword went berserk again as a result of Kallen's will being weakened.
After the trio defeated Jizo Matama again, Higyokumaru offered Sakura that she can resurrect her consciousness to the real world using the Jizo sword. Sakura then declined, and implied that staying in the Stigmata dimension with Kallen was just enough for her. The Jizo Mitama sword was later thrown in the real world, and Kallen, Sakura and Higyokumaru accepted their lives in the Stigmata realm.
- The genes that made Kallen's clone "A-310" or Theresa Apocalypse was hers and Vishnu's, an Emperor-Class Houkai beast with accelerated regeneration capabilities.
- Otto Apocalypse's first geniune love was Kallen, as she was his only friend who could understand him. However, he never got a chance to tell his feelings as Kallen expressed her love for Yae Sakura.
- She shares her name with Kallen Kozuki from Code Geass.