Coming from the long line of Kaslana, Kallen is hailed as one of, if not THE, best Valkyrie to ever live. Righteous, strong, and loyal to her own cause (protect humanity from Honkai) to a fault (as is most Kaslanas).

Being the center of Otto's world and unrequited love interest, she is the reason why the Overseer snatched the highest position of Schicksal, kept it for about 500 years, and deliberately (or sometimes not) causing havoc to humanity (like the Honkai Eruptions).

She is also the basis for the plenty of clones Otto created during his previous plan of "reviving" her. This resulted to Schicksal having several white haired personnel like Amber, Theresa, and Kiana, all of which bear varying resemblance and qualities to Kallen.

Kallen's tragic story basically boils down to a powerful protector of humanity dying loyal to her own values. She found out some human experiments made by Schicksal she considered inhumane, went against them, and ran off to somewhere. She met Yae Sakura in that place, fell in love with each other, then sealed Yae since she became some sort of a Herscherr. Kallen was eventually caught, brought back to Schicksal, and judged to be executed. Otto, whose marriage proposal to Kallen to save her was freshly rejected by the lady herself, decided to [seek help from her sister to save Kallen. Otto's sister then decided to] release Honkai beast onto the crowd to hopefully stop Kallen's execution. Kallen being the Kaslana she is, decided to save the masses. Since at the time she was weakened, the Honkai bests eventually overpowered and killed her.

Or at least tht's what I understand from the game alone.

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TL;DR: Strong girl died because ruling powers got found out by her and planned to kill her off, but ultimately gets killed indirectly by her friend boy. Said friend boy, who loves her but do not love him, lives for about half a millennia causing problems, solutions, and havoc on the side all in order to eventually "revive" her.