So, for people who haven't noticed, this manga has a horror undertone with things that should be worrisome but are played for laughs, like the kappa's biologically intrusive water control, Sam Adam's literal omniscience and Mizuda's ability to create life. She already openly said in Ch. 2 that she isn't an octopus, and if you recall the original explanation of what demi-humans are, they're descendants of GODS. Also, Mizuda has always been older than Akigai. She's always behaved as the onee-chan in all their interactions from 10 years ago, especially in ch. 5.