I wanted to point out that in the Undertale art book, on Frisk's character page, the title for the page is actually 'Human', and the file names of the art are titled with 'MainChara'. The name Frisk is actually nowhere in this section, and I have to recheck every page again for sure, but it's possible 'Frisk' is never even named in the whole book. Personally this feeds into some of my theories, so I though others would like to know it too.
Undertale artbook anon here, I went ahead and checked through the whole book and I do not believe the name Frisk ever shows up in the book, even on the protagonist's character page. (also I have the fourth edition of the book, in case it is different in other editions). Just a fact I found interesting
(undertale spoilers)
The reason for this is surprisingly simple: Frisk’s name is a spoiler!
At the very start of the game, the player is able to enter a name to presumably name their character. However, it’s not until the very end of the Pacifist route that someone finally asks the human for their name, and it turns out their name is Frisk. This is a major spoiler for the game, which is why Frisk is always referred to as “The Human” in all official Undertale merchandise.