First I shall mention the unavoidable achievements that can be unlocked just by playing the game
"Cupid of love"
"Love at first sight"

If you feel I have made any mistakes in this guide then please let me know and I will look into it and correct it if needed :)

Mitsuki route achievements:
"Sharing a bed"
"Abomination" (may also be unavoidable)
"Bound Up"
"In the nick of time"
"Victory"
"Mitsuki"

Route:

• Try and flatter Mitsuki
  
• I think you'd be a better girlfriend Mitsuki
  
• I wanted to see you Mitsuki
  
• Let Mitsuki share your bed (Achievement - Sharing a bed)
  
• Nobody else can see you dressed like this but me
  
• Save yourself while you still can (Achievement - Abomination)
  
• Nah, I just like to make you blush
  
• I won't go down without a fight
  
• Try to appeal on Mitsuki's behalf
  
• Let Serra eat the omelette
  
• Look around the park with Mitsuki
  
• Just leave her
  
• I care about Mitsuki a lot
  
• Wait for Serra

Follow the route to the end for the following achievements -
"Bound up"
"In the nick of time"
"Victory"
"Mitsuki"

Serra route achievements:
"Good samaritan"
"Stuffed bear"
"Shelter from the storm"
"Serra"

Route:

• Threaten Mitsuki
  
• She might be
  
• I like the taste of your coffee
  
• Tell Mitsuki to sleep on the couch
  
• Your manager doesn't know what he's missing out on
  
• Save yourself while you still can
  
• Yes, you're super adorable
  
• I won't go down without a fight
  
• Try and flatter Serra
  
• Warn Serra of the unspeakable horrors of that omelette (Achievement - Good samaritan)
  
• Look around the park with Serra (Achievement - Stuffed bear)
  
• Invite Serra into bed with you (Achievement - Shelter from the storm)
  
• You should know you're the only one for me
  
• Wait for Serra

Continuing this route and finishing the story will unlock the "Serra" achievement

I have previously made a guide for this ending:

https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1301795961

This route can be unlocked by choosing to go back to heaven with Serra when she comes to take you back. This route will unlock the "Bad girl" achievement.

If you'd still like a guide on this then I shall provide an example:

• Try and flatter Mitsuki
  
• I think you'd be a better girlfriend Mitsuki
  
• I wanted to see you Mitsuki
  
• Let Mitsuki share your bed
  
• Nobody else can see you dressed like this but me
  
• Save yourself while you still can
  
• Yes, you're super adorable
  
• Go back to heaven with Serra

Continue with the rest of the story to unlock the "Bad girl" achievement

As far as I know, you can get this ending on either Mitsuki or Serra's route.
To get this ending is to not wait for Serra when looking for Mitsuki.
I shall provide a rough guide anyway if it will be of help.
(This is Mitsuki's route but the end choice is different)

• Try and flatter Mitsuki
  
• I think you'd be a better girlfriend Mitsuki
  
• I wanted to see you Mitsuki
  
• Let Mitsuki share your bed
  
• Nobody else can see you dressed like this but me
  
• Save yourself while you still can
  
• Nah, I just like to make you blush
  
• I won't go down without a fight
  
• Try to appeal on Mitsuki's behalf
  
• Let Serra eat the omelette
  
• Look around the park with Mitsuki
  
• Just leave her
  
• I care about Mitsuki a lot
  
• Go ahead without Serra

Finishing this route will unlock the "Good Kitty" achievement