No, it's the easiest candidate for the parasite. It could be something that's not directly been shown yet, but mentioned. It could be an outsider, and be the reason Harry can harm them the way he does. The parasite could be anything.
Ocam's Razor usually do it.
If you get the time to check the WoJ's, (you can find most of them on Serack's thread, and the newest ones at the thread he ask help transcribing them, or with the forum search device, just take effor, if you really want to read you can find it) you will find WoJ's saying that
Both Lasciel and Lashiel appear in GS. I belive one interview Jim said Lasciel, the other day he confirmed Lashiel, and I don't remember correctly but also there was one he mention both of them.
And there is another WoJ that Lashiel appears near the end of the book. That leaves her with only two options, Parasite or Whispering Shadow.
I ruled out Lashiel as the Whispering Shadow, cause she dosen't need a shadow to whisper to Harry, she's inside his head, also would be aganst her interestd to have him killed.