I haven't personally played Undertale, so I can't speak to much of the stories content, but I can offer some feedback on some technical aspects to your writing.
First, I would recommend checking over your writing for typos and small grammatical mistakes before posting. There are a few spots where the word at the start of a sentence isn't capitalized, or a period is missing from the end of a sentence. There's also a part in the third paragraph where the beginning of some dialog is marked with a quotation mark ("), but there isn't one at the end. There are a few other spots were dialog is not marked. These are all small errors, and easily fixed, but they can also slow down the reader as each will take them out of the story. It's good to get in the habit of reading over what you've written before sharing it.
Speaking of dialog, I'd also encourage you to try and use more! There are several spots, including in the first paragraph, where you say people are talking, but don't write the dialog:
they approach to Meena and introude herself as Alphys. the new caretaker of this place. She asks Meena for her name. Meena respond her name and ask where is she. Alphys tell her that they are in the RUINS.
I think writing out the actual dialog for these two characters would improve the story! There are other spots in your story where dialog could also be added in. If you don't feel as confident writing dialog, I found a pretty good guide on the grammar and structure of it:
https://self-publishingschool.com/how-to-write-dialogue/
The last bit of feedback I'd give can be better explained by many other people, so I'll let someone else do it:
https://www.youtube.com/watch?v=Lqk_a5LfR5Y. The main point of "show, don't tell" is to include more details, take the time to describe your locations, characters, and the actions they take.
