Spoilers ahead for the choices that lead to each ending and the theme of each ending!

Ending 1/Mini Bad Ending: during the soul connection on day 3, click "Grim... ma—maybe this is a bad idea...", then "I... I think something's wrong... this doesn't feel right...", then "Please... stop..." 

Ending 2/Full Bad Ending: Pick more negative choices throughout the game, don't water your plant on any of the days, then on day 6 choose: "Vaguely, yeah." just before the first call, and "I don't mind. I'll take the sickness. The taint."

Ending 3/You Win: flirty all the way through, then on day 6 choose: "You really know how to sweep me off my feet.", "You're cute. Cute and stupid." and "How do I stop? What if I give it all away?!"

Ending 4 (DLC)/Both Win: pick DLC choices throughout the game, then on day 6: "Do you like me then, Casper?, "If I let my guard down the reaper will steal me away~", then "Maybe I'm a new class of reaper?"

Ending 5/Wedding: joke about a wedding throughout the game, then on day 5: "Want to meet the in-laws, Grim?", and on day 6: "I feel touched!", "How about a shotgun wedding?"