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 After falling down in the Underground she had met Flowey who helped her on her journey. It was alright for the most part until she had died for the first time, normally there would have been the ability to SAVE/LOAD but there was none the only way for her to come back was to RESET each time, however, these resets would only happen upon death. These are not controlled.

After loading the human soon found herself with a buttercup that had grown on the left temple, trying to remove it felt like ripping out her own flesh. Despite this new growth, Frisk continued on her path in the Underground. Many deaths happened just in the ruins alone and even more so with Toriel as she attempted to force Frisk to stay. That did not last too long and soon gained her freedom. Then...she met Sans...Flowey had told her not to trust him but did and shook his hand. He killed her with the buzzer, after the first death, and many thereafter he started to understand and decided to help her reach the barrier. They both understood what would happen when reaching the barrier and despite all of that they trudged on. Sans regretting everything that he's done and Frisk telling him that things will be okay. She reminds him that sometimes kindness is all we have, sometimes kindness is enough.

At this point, she is half-blind (or fully blind depending) due to the flowers covering most of her face. Her clothes hide the rest of the "garden" (growth). The curse is caused by the leftover hatred for humanity that harbored within Chara. This hatred is a thick black ichor that latches onto the soul and drains the human (host) until they die or until they are killed off by the monsters of the underground. The extent of the curse wasn't known until Frisk fell down. When it latched on it blocked off the full use of determination forcing resets and causing buttercups to bloom.

It hurts whenever she does feel determined or uses determination and the pain is caused by the ichor squeezing the life out of her soul. There is something else, if a reset were to become glitched she may end up in a different AU.