It's spoilers because it shows that contrary to Aqua's defamation, Eris does not pad her chest, which we will learn in the future. Seriously though, I have no clue why it's marked spoilers.
I think the spoiler is Eris and Chris are the same character except that this artwork shows otherwise so I am not sure if spoiler tag counts here.
The spoiler tag applies because the dual persona tag applies. This picture depicts Chris and Eris existing side-by-side as two separate people, like a picture of Bruce Wayne and Batman; hence the dual persona tag. The fact that they are one and the same is itself a spoiler, hence that tag.
