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I don't think there is no real checklist for the game, but you can romance all the girls and get max stats. Other than all of that, you can use the explore mechanic and find extra side girls for more scenes. Also the Act 2 finale isn't quite finished yet so I recommend having a save file prior to activating the finale so you can still go back. 

What bloodnguts said :) There's no official walkthrough (there is a wikia, but it's not consistently updated) but the community is very active over on the CK Discord server and are always happy to discuss secrets and side content :)