So this is what my idea of rubedo could be , i know this is not gonna be correct but i just wanna do it anyways. So here's my idea behind my concept
Since Rubedo in Latin means redness and it was adopted by alchemists to define the final stage of their magnum opus and we all know that Tabula Smaragdina is an alchemist so it was make sense. Oh and for some reason the philosopher's stone is associated with the color red i think that rubedo was meant to be Tabula's philosopher stone ? Oh and Nigredo calls her Spinel and most spinel gem is known to be red.
The fact that Nigredo dislike Rubedo and albedo thinks that she's cute means that Rubedo is gonna be bad news, knowing that Tabula is also into that gap moe shit. i though she would most likely look like a beautiful young girl while hiding her more inhuman parts under her most likely fake skin. The reactor core at the center of her chest is where the Caloric Stone could be put in to activate her.
Since Rubedo is the strongest close combat specialist there is in nazarick i though i would give her the nintendo power glove since it's a tool that only players aka "supreme beings" use to inforce their will upon the npc's so call "World" ...also i dont know anything else that could fit so yeah ...