I figured out what the Fist of the North Star is~ Thanks!
Also I think I might've found a bug - Back when your reply to my comment didn't exist, I loaded up a new save file (max stats and money, if that helps) to try to find out what the Fist achievement was - and I found out what string theory is (Res. check 110 ;) ) - I didn't get the achievement from that. I just checked the Fist one on my new save and it works fine, and Fawn's replay doesn't work so I can't replicate String Theory's conditions again on that save.
Also also, another interesting thing - a bit after Fawn's harsher play (String theory attempt), when I slept (at the place where you can pay 5C for a homeless guy to guard you) I got a strange dream - My chara was apparently on a beach with a chastity cage trying to touch themselves - they remembered they put the key at the resort they were staying at - and a weird sort of half image with what appeared to be a version of Fawn's basic features flashed on screen for a frame before the dream ended. Is this a preview of the upcoming dreams functionality? Or is this some extra rare occurrence from the base game? I can't find anything on the wiki, so it might be worth investigating if this is something new. If I can replicate it and record it, I might edit this comment with the screenshots.
It does seem like the string theory achievement broke when I went to port the mod to the latest version.
As for the dream it is from the mod. It looks like I forgot to disable it after I was done testing. That dream sequence probably won't be done for a bit, but the first dream will be in the next update.
I'm going to try to get a bug fix out in a few days (or tomorrow if the string theory problem is simple to fix).
Thanks for reporting :)