Given how these nikke artists (dfm (darknessdfm), chela77, nasaca, yoya yogurt, boninit, bumcha, jam (nandade), tigermtang, mikan03 26...) post the claims, I would assume this one is also game screenshot due to the low resolution (it's not that low but you can extract at a much better res)

If you actually open game bungle assets you should know that only the story cg can be extracted to clean PNG files. Everything else: character, event intro background, burst animations, favorite item cutscenes, ... are spine (medium). And for some reasons Shift Up's policy prevent the artists from sharing the full res exported pics for these spines. Without spine license or whatever internal tool shiftup uses, you won't be able to display all the spine layers together correctly. So the artists can only do game screenshot

I was asking someone else about tagging this post with dfm (darknessdfm) because he's the original artist of Naga (Nikke) default art. But this skin is redrew quite significantly to me