I went back and looked, and you are absolutely correct. I checked the Rook city kickstarter as well, and what you said holds up, with one VERY important caveat.
The base artwork for all promos, new heroes, and new villains doesn't have HP or anything else listed. it is just the artwork. However, in every instance I could find (links to follow) where it was more than just the base artwork- HP was listed.
For instance, Ra's Horus promo was initially teased here (base artwork, bottom of page): http://www.kickstarter.com/projects/gtgames/sentinels-of-the-multiverse-infernal-relics-and-en/posts?page=8
Here is Ra's full card (artwork + formatting and text): http://www.kickstarter.com/projects/gtgames/sentinels-of-the-multiverse-infernal-relics-and-en/posts?page=6
compare Dark visionary as base artwork to Dark Visionary with formatting: http://www.kickstarter.com/projects/gtgames/sentinels-of-the-multiverse-shattered-timelines/posts?page=2
What I see with the Assistant is that the card has a subtitle, text, even the nemesis circle. It is NOT base artwork. It is in every respect, a fully functional card, just one without HP. To me, this suggests that the Assistant has no HP to list.
Ambuscade has a mechanic where he cannot take damage, but even on the side of the card that renders him immune to damage, he has HP.
While I could be wrong, and certainly have been in the past, I do not believe that the Assistant has HP- at all. that then suggests to me that there is an alternative way of beating her (some sort of plot the heroes must resolve), or she is someone who is to be played in conjunction with a standard villain.
Obviously, time will tell. And I'm quite happy to say that I'm wrong (and crazy) if I'm off-base.