Ellie was one of the Hostiles that we saw in 1954 when they stumbled into the former army camp. I had to look her up, too. Lostpidia link:
Ellie - Lostpedia Suffice to say we know virtually nothing about her.
Some people think that "Ellie" is a nickname for "Eloise" and that she, therefore, is Daniel Faraday's mother who we know in her older, matronly form. At this point, though, that's pure conjecture (even if it sounds good.)