Cheth's true, Whole Healthy form is a giant bonefish, right? If so, I'm just confused about one thing: as early as the prologue, we see these disembodied eyes for Cheth. These seem to be part of his true form, as this clearly isn't "wearing souls as clothing," just as the red veins are indicated to be part of his true form. So my question is: ...why are those disembodied eyes so HUMANOID? They certainly don't look like fish eyes.
Quick answer: Because I like it and it's fun.
Long answer: Cheth can do weird magical illusion stuff with his powers/veins to some degree. Like when he changed the painting in Phaedra's quarters. How did he do that? Did he paint it over with haunted water? Maybe? Probably. It's also because I like the idea of it and it's fun. He's also still quite attached to his human forms, so there's no reason why he would restrict himself to bonefish eyes. And his bonefish form had these big purple eye fans anyway. So the purple eyeshadow is a nice proxy.
tl;dr Rule of cool bc Cheth is literally magic water spaghetti