First off, Princess Serenity was half-demon (we never did find out who her father was...) by way of a fling the Queen was seduced into by an incubus. Getting burned in the aftermath is part of the reason she is distrustful of demons, and she kept the Princess's heritage a secret from everyone, even Luna. Serenity was killed during the final confrontation, which set off the Queen's attempt to use the crystal to seal away the Dark Kingdom. She reincarnates as Usagi, as per usual, but when awakened she takes on the form of a succubus (of the Morrigan Aensland variety, not the traditional dream-walkers). Her Serenity form looks a lot like Rozalin from Disgaea 2; her standard transformation... well, it was inspired by <a href='http://gelbooru.com/index.php?page=post&s=view&id=794041' target='_blank' rel='nofollow'>(NSFW!) this (NSFW!)</a>, but it probably won't be quite so revealing. On the other hand, you never know... I've always thought that if Usagi weren't tied down to Mamoru, the Senshi in general would probably be her lesbian harem; depending on how the story goes, that may or may not wind up getting explored...