Based on that alone, you still dont have much on who she is. So, I decided to go on the idea she may have been a Lancer. I went searching for the name of Lances/Spears of history. I found the following object...
Used by Izanagi and Izanami to create the Landmasses of the World. Still not too much evidence. Then I thought about it a little more. If you think back to the days of said God and Goddess, she had died and been sent to Yomi. ((Essentially Hell.)), Izanagi went to retrieve her, and was ordered not to look back for if he did, she would turn ugly. Sadly, he couldn't keep himself from looking back at her. If you think about how early in time this was, Izanagi could have found her ugly because she gained the horns, tail and claws ((I say claws cause her fingers may be such.)) She would potentially retain those features as a Servant as well. Now comes the part of her being a Goddess. Well, easily solved when you notice that Twice Summoned a Saver/Savior Servant, who happened to be a Diety as well. =/
I needn't say again, this is merely a theory I have.