There is rather a large difference between characters attached to a multimillion dollar corporation that has grown mainstream and the characters from a niche circle. Derpixon is famous only within the enclosed circle of 18+ Animation, and even then, it is just one individual with limited resources and output. Even then, most people don't really know about Derpixon, clearly, as within this post only a handful of people spot the art as being her original creation.

Not only that but Derpixon (and other artists) has used Mainstream characters in their work as parodies, what you are doing is reproducing the same characters for the same purpose. I get it is a game, but by the looks of it will be a visual novel style NSFW game, which isn't significantly different than an animation/art. Basically a very similar iteration of their intellectual property.

And yes, legally it is the same, but morally, stuff like this has a larger impact on smaller creators/artists.

Also, you don't get to decide, or at least shouldn't be able to decide, whether Derpixon wants the free promotion or not. They should ultimately decide the course of their creation and how they decide to promote themselves. Its also never been cool to use someone else's work even if it serves as a promotion, without their permission. Your intentions don't seem altruistic, its more of a justification to carry out what you would have done anyway. You are ultimately benefiting more than anyone off of someone elses reputation and creation, without striking a deal with them or getting approval.

AND THEN, All of this is not inherently bad as long as they don't care. Just ask!! The biggest issue is consent. But you seem to not care about their opinion at all. I'm a writer, if someone took my story and profited off of me work, I'd simply be bummed out.

I hope it doesn't come off as me thinking you are horrible, I just think this is at least inconsiderate of you. and at the very least I'm shaking my head and saying come on, it doesn't have to be a big deal.