On one hand, the pixiv tags have it marked as original. On the other hand, this artist has done other Neptunia works before other people besides the artist can add tags as well on pixiv so maybe?
The post title includes the phrase "うちの子" (uchi no ko,) literally "my child" but essentially translates to "my OC" in this context.
In addition, on Pixiv, you can tell which tags were added by the artist using the dialog that pops up when you click the plus sign next to the tags. Tags added by the author have a "locked" icon next to them (meaning you can't remove them even if you think they're incorrect.)