Thank you for your three answers. If I may ask, it seems your opinion of me changed slightly between your different answers. Do I still come across as a shitty person?

And I insist that my decision hasn't been made. Karen is my ideal as a woman, not necessarily as a partner. You may very well find that a sculpture is the rendition of the ideal male or female body for you, that doesn't mean you'd like to marry that sculpture, does it?

I admit that it is possible we wouldn't be compatible on other levels that we currently have no idea about (sex, daily routine, etc).

you are emotionally cheating on Lisa

Yes I am, and I can control myself to ensure that I'll never physically cheat on her. That I am sure of. With Karen or anyone else. I do not believe it's possible to master our thoughts on a subconscious level. Those who told me "you need to forget about Karen" do not understand how things work, had I been able to do so, à la Eternal Sunshine of the Spotless Mind, I probably would have, but it's not a possibility. And I do realize she's a threat to my relationship with Lisa, that's the whole reason why this thread exists in the first place. But it doesn't change anything because whatever I do, I cannot forget her. So no, I cannot fathom having no relationship of any kind with Karen.

Really, every comment in this thread made me think a lot. In the end, I think that as Asimov would put it, currently there's "insufficient data for meaningful answer", I admit I could be infatuated with Karen. I do think that the constraints of a reddit thread didn't let me fully explain how much we could have communicated since we know each other, and on what level we did so. But for the sake of common sense, to be a reasonable man, I won't rush into any kind of decision. I'll make sure Karen and I know each other better, that we spend time together, and if my feelings stand the stress of time, then I'll know what to do. Because then I'll have no other choice. And then, I definitely hope I won't hurt anybody, and that everyone gains something from my interactions with me.

Just one last thing: putting words on feelings is never easy, and when it's not in your native language it's even harder. Subtle differences are lost in translation, which further complicate the matter. Anyway, I'm sincerely grateful for every answer that I received, and all were useful. Thanks especially to you for your brutal honesty, it was very much needed. Have a nice NYE!