This Ellie is one of the first batch of little sisters that were converted by Tenenbaum before the ones we all know and love came into play(kinda like the first drafts of test subjects, when they were still in the trial-and-error phases). Her slug didn't attach itself properly, messing up the biological chemicals that connects a big daddy + little sister together AND making her age faster than the other sisters. SO she was considered a failure and thrown out by Tenenbaum instead of being mercy killed(Tennan. knew she would feel guilty killing Ellie so she just pushed her away). Before Rapture fell she was basically a homeless adolescent who roamed around the shadows, never being able to find a home or friends due to how she looked(in my headcanon, the sisters were a secret project while they were still being figured out, then were announced to the public later).

-Her dress is the one she was given when she was kicked out but it's gone through some fixin's to make it still fit her properly(scraps she found from dresses laying around)
-Is still able to regenerate health and collect adam(which she does for Jack), but at a slower rate than the others sisters.
-Always trying to hold Jack's hand when they're walking through Rapture(he usually lets her).