All right, I think I'm basically done mechanically? The starting equipment is split between the two
[url=https://www.myth-weavers.com/sheets/?id=2867343]Aelyeth[/url] 
[url=https://www.myth-weavers.com/sheets/?id=2867347]Eidolon[/url]

I'm not that familiar with 5E though, so hopefully I got everything correct.

Anyway, about the backstory...as I mentioned, originally Aelyeth was a character for a 3.5 Eberron campaign that never got started. The idea was that she worshipped the Spirits of the Past and thus was wanting to bind an ancient hero as partner ala Fate's Heroic Spirit, she made a mistake and ended up binding a fiend instead. How much of that would fit for your setting? [@ShiroKiyoshi]