First off I'd like to start by saying none of my facts are real or verified by bungie this is 100% theory. Let's begin. Crota is green, as we all know, and oryx himself is a redish brown. This means oryx "hooked up" with a female that was most likely green. Where do we see a green female entity? Eris herself. (Yes she isn't green herself but generally when you think if eris everything is remotely green). I hypothesize that its possible whatever race oryx is, the female counterpart is green, resulting in the birth of a green offspring. Crotas a boy though so this wouldn't work but maybe Eris is a completely separate species from oryx which would result in crotas color and shape. Now I suspect oryx already had knowledge of guardians and the light and knew one day they would come for him so how better to destroy something than from the inside? So oryx needs someone on the inside and eris volunteers. She couldn't possibly fool anyone looking like an alien so, much like crotas soul in the crystal, Eris's alien side is extracted and put into her rock. This could explain Eris's overly generic facial features. Eris's eyes are also covered but if you look at the shape of her eyes they are on par with the shape of crotas eyes, being why she would cover them with the rag. (Side note: no matter where eris goes she always has this rock with her so its possible that it has sort of a physical bind to her because its part of her soul so she can never truly separate from it). Being an outsider in the vanguard its pretty obvious that eris is the odd one out and never really accepted so really it shows a clash of light and dark because none of the vanguard take her too serious. Now why would crota go into the hellmouth to kill, what could be her son. Think of it this way, she was assigned to the raid. Now when a single guardian dies, crotas over soul is summoned to take the rest of the guardians, or their light per say. How did eris survive the over soul? Pretty simple , she has no light to take so she simply walks out of the raid and no one is the wiser. A more weaker point as to why eris could be connected to oryx is how the guardian is able to make it to the dreadnought with Eris's ship. Perhaps this ship was given to her by oryx, which could explain why similar ones drop in the crota raid, or it was just the ship she has from the beginning. Now the ship doesn't blow up until the ghost pulls the guardian out so perhaps when the guardian occupies the ship its recognized as eris but when the life form is pulled out of the ship, it becomes a floating piece of space debris that gets vaporized by the super weapon. Finally, eris sells all this unusual stuff and takes in materials from the dreadnought. It is said that our guardians are the first to set foot on so how else would eris have these items if not taken from it herself. I thank you for reading through and hope you guys got knowledge, interest, or even some amusement out if what I've had to say. Stay safe guardians.