For those who STILL are asking wtf is Zeta, this is the answer.
This thing is the Unit R Reborn Capsule, yes, this capsule transforms
a dead body in a R Unit. Yup, you read well, the R Prototypes are undeads.
Thats why the project have the R, this "R" means resurrection. A project
to transform a dead person in a combat cyborg unit with the most advanced
technology that even Doctor Eggman its unable to understand it.
This laborious, slow and very expensive procedure replaces all the skeleton
of the experimental subject for a mechanical skeleton made in a special
titanium, all the blood is drained from the body (making the skin of the R
Unit changes his color apperance during the procedure) and the heart is
replaced for the Generator Lazarus, a generator who have in his inside one
of the Chaos Emeralds, so, all the mechanical skeleton is powered by the
power of the emerald, depending of the emerald, the eyes of the R Unit
changes his color, so, you can recognice quickly what emerald have inside.
But thats not all, the Generator Lazarus is capable to maintain an organic
organism alive, doing the job of an artificial heart with a special blood,
more dark and dense, capable to regenerate and cure wounds and injuries more quickly. Anndd.. the last thing.. no, Zeta dont have a metallic skin, shes soft like any other girl, just her skin changes the color during the procedure before mentioned to a gray (purple is her original skin color), and she have all her original internal organs, excepting the heart, replaced for the Generator Lazarus.
So, she have to breath, eat and.. well.. all of those things the humans (or mobians) do, that make her a CYBORG, not a robot or an android. One last important note is the skeleton, made with an special titanium, the magnetism, electricity or corrosion doenst affect her structure, so, sorry for those who tries to capture her with a giant magnet, but thats not gonna work. :3