Update: I deleted ModSaveData.json and started over, this seemed to fix the issue of Kyu not being dateable. Unfortunately Momo does not appear after trashing the Goldfish plush in the park (as it is indicated in a differente comment), and therefore Venus also does not appear.
Both of them load without any issues (their sprites and voice lines seem to load just fine) when encountering them during the Nymphojin fight and endless dates.
As mentioned before I have installed the lastest version of the Ultimate mod and all the mods mentioned in the comment above.