Since this seems like a juicy expose thread I'll join in on the fun

The lineart is too similar, it even matches up for most parts of the hat and at other times he simply linetraced around the already existing outline.
The colors are super close to the point where it is obvious that Nathan has colorpicked them over and slightly changed them.

Here's some evidence with the lines overlapping and I've also picked out some of the colors which are almost identical to eachother.

GIF:


PNG:



Also Lyvia, if you are confident about him creating original content then please post the rest of his work and perhaps this'll turn out to be even more enjoyable than it already is