Many years ago the Doctor Fionna Marshall was a recognised scientist that gained her popularity due her discoveries and experiments about mutation and genetic modification that were a giant advance for human race, then suddenly the age of monster girls began. Many of them used their abilities to destroy cities and kidnap humans, also people that admired Dr. Marshall stopped paying attention to her since people thought monster girls were more interesting about her boring genetics experiments, this made Anna fill with anger and with an indescribable hate to everyone, hating monster girls and even her own race: humans.
Today almost no one remembered Dr. Marshall, not knowing that now she finished her most ambitious experiment hidden from everyone, so after putting the serum in the syringe and injecting herself she sensed many weird feelings through all her body. Her body remained the same, but essentially she did a major improvement that could defeat everything in her way. Her new ability was to absorb the genetic code of every living being she eat, being able to replicate parts of the creature in her body to fully transform into the creature she absorbed, but she could return to her human form every time she wanted, also made her mouth, vagina and anus stretch to almost impossible ways, this for being able to eat everything she wanted no matter the size. Now Fionna's goal was to absorb the DNA of every monster girl and assert her dominance to everyone, turning into the most powerful girl monster to get her revenge towards the beings that took everything from her.
After gaining her new ability, she decided to test it in her small lab, having a long vine hanging in one of her walls, after she ate the vines, part of its leafs and flowers she felt her stomach digesting almost instantly what she just ate and recieving all its genetic information. She couldn't believe it, her biggest experiment worked! To proved she got naked and changed her appearance, her hair turned green the same as her skin color, every single piece of her hair was replaced by long vines and her nipples turned into blooming flowers. She looked herself at the mirror admiring the wonderful creature she turned into, then the changes reverted and returned to her human form, ready to start her plan.