Sorry for the long ass post, but I've had this theory about where Sinful Lust(or the remake) could go and it's likely just me talking out of my ass but I'd like to share it if you guys don't mind. Hope it's not true, but I'm thinking in the end the "big bad" responsible for all that happened to Lizana could be none other than Diana herself. Hear me out.
I think Zetto is well aware that SL took a turn for the worse when Lizana started getting blackmailed and raped. Well let's say it's revealed that Lydia(the cop lady) didn't do that just because she was jealous of Liz back in highschool, but rather because Diana hired her to do it. We also know by some older sketches(that may or may not be canon, but let's just assume they are) that Diana knows Maria *very* well, so what if she was the one who also told Maria to start sleeping with Lizana's husband. And of course we all know Diana will seduce Yuki soon too, so that could be part of her big plan too.
Naturally you may wonder why would Diana do all of this shit to her own sister? Maybe they had a falling out in the past? Or maybe since Diana is the older sister and expectations for her were much higher, so she had to make a lot of sacrifices to get to where she is, while Lizana was allowed to live her "dream life" instead? And now Diana wants to prove a point to her younger sister that she only had that life because of her or something and she can tear it down whenever she likes, who knows. But I'm sure we'll get some backstory for the two of them sooner or later.
Another thing and this is pure headcanon, but Zetto often draws Diana to look a bit intimidating, and early on whenever he would post images of her, he always made sure to add a black heart emoji, something he never did for Lizana. May not mean anything(again), but I thought he was clearly implying Diana just wasn't a very good person and maybe in the end it's revealed this whole thing isn't about Lizana's but Diana's... sinful lust. Gosh, that was awful.
Anyway, as you can see lots of rambling and "maybes" and I hope none of that turns out to be true since Diana is my most anticipated character, but I think there's a reason Zetto waited for so long to bring her in and it can't be just to have her seduce Yuki and possibly Ramon and then leave. She's clearly meant to be an impactful character no matter what her role turns out to be.
Hell, this is an NSFW comic, not a fanfic, why the fuck am I overthinking this.