For those that never read the WN or LN, she was the one that chose to start as a zombie because she was drawn to that race more than the other human and inhuman races. Hell, even the plot says that she CHOSE to be a zombie, she wasn't forced to GOD DAMNIT!
It is considered more difficult to use because some inhuman races got clear disadvantages, like not being able to talk (which is extremely important for the NPCs), being animals or even plants, etc. In exchange for clear disadvantages early game, they receive the ability to evolve and become stronger faster, can change their abilities and some can even get extremely rare evolutions.
Now, as to what happened next, she chose herself to start in the catacombs because zombies starts with an horrible rotting smell (and beast people or animals have enhanced smell so you understand), a weakness to the sun and their rotting bodies.
She learns how to use the skill system that can add up to her already low stats and become stronger everyday.
Do you realise how much info got cut and changed? All that was from the first and second WN chapters. And the manga changed all that with a "Bug". And I won't talk about the fact that she seems like a battle junkie when in the LN, she's calm and doesn't really like beating the shit out of people. She even refuses to take part in a tournament later in the story even with her power that was already at that moment pretty impressive.
It's not a good adaptation so far of the LN/WN and that's already bad in itself. Now, I'm gonna read a bit when it will be translated but that's not good.
