ALL ENDINGS:
ending 1, semi bad ending. It's honestly the fastest to get. Choose whatever answer you want for the first two days, but during the soul connection on day 3, choose the panicked options as in: “Grim… ma—maybe this is a bad idea…”, and then “I… I think something’s wrong… this doesn’t feel right…”, then “Please… stop…”. The story will finish and you'll get the achievement.

ending 2, you lose the bet (bad ending). Be distant and dismissive, and pick more negative choices throughout the game, don’t water your plant on any of the days. don't open the window to get the flowers until it's your only choice when he shows up with the bouquet, then on day 6 choose: “Vaguely, yeah.” just before the first call, and “I don’t mind. I’ll take the sickness. The taint.”. The story will go on and you'll get the achievement.

ending 3, you win the bet. Be flirty all the way through, do your worst girlie, 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?!”. Once the story is completed you'll get the achievement and a super cute CGI of Grim.

ending 4, dual reapers. Since it's the DLC ending, it doesn't give you an achievement. If you decide to buy it, just choose DLC choices throughout the game, you'll recognise them immediately because there's a ★ next to them, for example: "Do you like me, Casper? ★"

ending 5, wedding. As soon as you suggest what's going to happen if he loses the bet, choose "Your hand in marriage." Joke about a wedding if given the change within the next day. On day 5, choose: “Want to meet the in-laws, Grim?”, and on day 6, choose: “I feel touched!”, and then, “How about a shotgun wedding?”

ALL ACHIEVEMENTS

First Chance at Death: this achievement will pop up as soon as you are finished creating your MC.

The First Day[...] The Sixth Day: you get these achievement as soon as you complete your day in game.

My Little Baby: interact with the pet you choose.

Serenity: water the plant on your bedtable, once is enough for this achievement.

Blooming Flowers: you'll need to water the plant once every day. It's easier if you do this while following any of the good endings.

Untrustworthy: you'll get this achievement once you get ending 1.

Take My Soul: you'll get this achievement once you get ending 2.

Dead Roots: do ths while going for ending 2, don’t water the plant at all. check on it on day 6 and on the last day before chatting; if you click on it by mistake during the previous days, choose not to water it.

Sealed with a Kiss: you'll get this achievement once you get ending 3.

Our Wedding: you'll get this achievement once you get ending 5.

Eavesdropping Champion: interact with the door every single day, and always choose to eavesdrop.

Patience: wait for the very last day. check your Internet browser in game before starting the chat with Casper, and wait for about a minute with the browser open! you'll see a white screen and the text changing from something like "there's nothing here" to "okay, click here to get your achievement", you click on the text and it's done!!!