Thanks so much for taking the time to let me know about that :3 I really appreciate it!
In this instance, that person hasn't done anything wrong because I'm not the original artist for the sprites. They come from an asset pack sold by a Japanese artist called sasAichi, and once you've purchased the pack, you're allowed to do whatever you like with the sprites.
What this person has done is just take the original sprite directly from the asset pack and use it without making any edits to the original version. Whereas I always like to edit any assets I've purchased where possible, just so that they don't end up looking exactly like other people's if they get used in other projects x3
I just used GIMP to edit all of the colours to something different and more fitting with my project :3
This is a little VS thing I made a while back when someone on Twitter asked me about the sprites and the artist who created them. It just shows the difference between my edits and the original unedited version. It's great that you picked up on that and took the time to comment though because if it were theft of some sort, I think any dev out there would really appreciate being alerted to it in order to try and do something about it. Thank you again :3