Well, a fully fleshed out response would take much more energy and time than I have at the moment. However, for starters, I would call your attention to the Katie/Liv scene that caused so much consternation when it came out. (Personally, I hated it at the time.) Where did that scene take place? It took place in Katie's dream room that is explained she
created as a place to retreat when she needed time alone. Except for the time when MC visited and mucked around with things, she says it remains pretty consistently the same. (Take away is that someone's creations/constructs can be mucked around with by someone else with power.)
Inanna dumps a shit-ton of lore on MC including some information about temples, places within her dream where people who travel there are safe and extra powerful if they have power. I think Katie's room is her temple. Having a temple itself may not necessarily indicate that she has any power since Inanna says anyone who enters her dream has one. However, if Katie has any power at all, it should be at its greatest when she is in her temple.
What happens after Katie's tryst with Liv in her room? When she wakes up, she begins acting differently. She's no longer afraid/nervous. She feels overwhelming guilt at how she treated MC and despairs that he could ever forgive her. However, with some encouragement from Alice, she goes in search of MC to at once punish herself (?)/prove to herself she can protect herself/confront MC and apologize and make amends. What on earth gave her the strength to do this when the day before she was basically afraid of everything, probably of herself most of all?
So, my theory is that Katie has a little bit of power, not much but a tiny smidge. (She presumably has the blood like Alice so that shouldn't be too hard to imagine.) She was able to find/create her temple in Inanna's dream, but she cannot create her own constructs/golems like MC and Alice can. She cannot directly impact her physical appearance and abilities like MC and Alice can. (Or can she? The game explains that she works hard for it, but damn, she always looks good no matter what she is wearing or what she is doing. Maybe she does influence her body a bit subconsciously?) She cannot heal her own mind like MC is able to do for Maria.
Since her little bit of power is maximized in her temple, I think she was able to use MC's subconscious/ability through his construct (Liv) to heal her own mind. Why through sex? Well, it's a porn game so why not? However, it also seems that an awful lot of Katie's hangups/issues are sexual in nature. It allowed her a safe lesbian encounter which seems to be something she was interested in experimenting with while at the same time interacting with what is essentially a piece of MC's subconscious broken off into an autonomous (?) being. It should be clear that all her protestations aside, she is very interested in MC. This all kinds of meta, but the threads start looking like they might form a pattern. I'm sure I could find other examples, but I think this is enough to get my gist for now.
Why might this be important to the story? Inanna and Eve completely discount her. If she has any power at all, it can be deployed at the opportune moment out of the blue. I'm not convinced either Inanna or Eve are truly evil, but there is definitely some kind of conflict between them that MC is destined to be in the middle of. Katie could potentially be a literal secret weapon in that conflict. This could get especially interesting if the nature of her power is the ability to use other people's abilities/constructs rather than having her own.
Of course, it's all just a theory in my head right now. I'm still waiting to learn more about Katie and what happened to her in Inanna's dream. I really, really hope PL is able to continue development of the game so I can find out what if anything I'm right about.
I hope you enjoyed my TED talk!