As a WMG subpage, all spoilers are unmarked as per policy. You Have Been Warned.
Eleanor is a form of Renelle.
The only clue we get to Eleanor's past is that Dr. Talbert is the one who made her pendant. With it being made of Remnant, Jake referring to her calling herself Renelle as a "half-truth," and her being able to recite what appears to be Renelle's actual backstory and recognize her house, I'm pretty sure that Eleanor is a separated Agony-form of Renelle. Somehow she got split from Renelle's body/Remnant and became a much more powerful threat than anticipated, with Renelle (who lost or threw away her Remnant-necklace) being none the wiser. This would also explain why she feeds on Agony (it's what she is,) desires Remnant (wants to be whole again/get more power), and targets the same insecure teens (what Renelle was when she was kicked out of home). Why she looks like Circus Baby, though, is anyone's guess. Maybe Renelle was a fan?