Ceroba Ketsukane Undertale Yellow

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This was requested by the one and only Superstar5

Hailing from the Wild East, Ceroba grew up in the Dunes with her childhood friend Starlo, the two becoming best friends later on and dating for a short while. However, Ceroba eventually broke up with him as she wished to "grow up", and he didn't. The two remained on good terms. She eventually married a Boss Monster named Chujin, and bore him a daughter named Kanako. Chujin would work under King Asgore as an engineer for the Steamworks, but after his prototype robot Axis nearly destroyed the king's flower patch thanks to a malfunction, he would be fired from his job. Despite this setback, Chujin would continue working on Axis in secret, telling his family that he simply resigned instead.

One day, during a visit to Snowdin to meet Kanako's friend Dalv, when Ceroba and Chujin weren't looking, a human arrived. Hoping to protect Kanako, Dalv attacked the human, but fled when the human retaliated. Enraged at how his daughter's life was endangered, Chujin ordered Axis to hunt down and apprehend the human. Thanks to a glitch, Axis killed them instead, leaving the Blue Soul in Chujin's possession.

Now believing that all humans were dangerous, Chujin decided to make a serum capable of making normal monsters as powerful as Boss Monsters. To do this, he needed an extract formed from fusing a Boss Monster soul to the human's. Lacking other Boss Monsters to experiment with, he decided to test the process on himself in secret, documenting his progress in secret video tapes. Unfortunately, due to its violent nature, the blue soul repeatedly rejected the fusion, causing his soul to deteriorate and leaving him fatally sick. He made his final request to Ceroba in a videotape revealing his experiment to her, asking that she'd continue his research: seek out a 'pure' human soul - one untainted by violence - and find a other Boss Monster to test the serum on. He specifically requested that she not use Kanako in the experiment, so she could live a happy life, free from the pressures of his experiment.

While she initially followed her husband's last request to the letter, Ceroba soon became desperate thanks to a lack of Boss Monsters in the Underground. One day, Kanako discovered and watched Chujin's research tapes. She also found a document in the trash - one made by Ceroba - stating that she believed she had found a way to purify the soul extract, presumably making it safe. Despite seeing her father's final wish, Kanako volunteered herself to be a test subject. Running out of options, Ceroba agreed to test Kanako. The extract rejected Kanako immediately, causing her to "fall down" (fall into a coma that precedes death in monsters). Ceroba would sent Kanako to Alphys, hoping that the determination experiments could somehow revive her. In the meantime, she would search for a human soul pure enough to form the serum with.