Alrighty, so this is a guide to getting all of the endings :3 I've tried to make it as spoiler-free as possible!

Leah's Story 

This one is pretty straightforward :3 The choices mainly change the dialogue you get + depending on what you choose, you get different versions of certain scenes/see different things. None of the choices impact the main ending though aside from the final choice.

Choice 01:
  Go with him – Only changes dialogue
  Stay at home – Only changes dialogue

Choice 02:
  Approve – Only changes dialogue
  Disapprove – Only changes dialogue

Choice03:
  Invite him in – Continues the story without visiting Aku's house
  Let him leave – Causes Doyle to become upset, leading to the same choice coming up again

Choice 04:
  Invite him in (2nd time) – Continues the story without visiting Aku's house
  Let him leave (2nd time) – Forces you down a path that leads to two endings

If you picked 'Let him leave' twice, you will end up at Aku's house.

Aku's house choice:
  Kiss him back – Leads to ending 1 'Our Embrace (A)'
  Shake him off – Leads to ending 2 'Our Embrace (B)'

If you invited Doyle in, the story continues.

Choice 05:
  Tell him to move – Only changes dialogue
  Let him sit – Only changes dialogue

Choice 06: 
  Kiss him – Changes dialogue and alters a later scene with Aku
  Slap him – Cuts the scene short + changes dialogue and alters a later scene with Aku + prevents you from getting ending 3 + locks you out of playing sexy scrabble with Doyle later and some of the other choices

Choice 07: – If you picked 'Kiss him'
  Stop him – Cuts the scene short and changes dialogue + prevents you from getting ending 3 + locks you out of playing sexy scrabble with Doyle later and some of the other choices
  Allow him to continue – Continues the scene and changes dialogue

Choice 08: – If you picked 'Kiss him' then 'Allow him to continue'
  Agree to let him stay – This leads to ending 3 'My Angel'
  Decline his offer – This continues the story

Choice 09:
  Hug him – Only changes dialogue
  Do nothing – Only changes dialogue

Choice 10: – If you chose to kiss Doyle + allowed him to continue
  Tell him to back off – Cuts the scene short and changes dialogue
  Encourage him – Continues the scene

Choice 11: – If you chose to kiss Doyle + allowed him to continue + encouraged him
  Tease him – Takes you to the word puzzle – Pick 'Something else' then enter 'Checkmate or checkmate'
  Protest – Takes you to a potentially slightly NSFW scene with additional choices

Choice 12: – If you picked 'Protest'
  Do as commanded – Only changes dialogue
  Question him – Only changes dialogue

Choice 13:– If you picked 'Protest'
  How about no! - Cuts the steamy scene short
  Bring it! - Continues the steamy scene

Choice 14:
  Answer it – Leads to ending 4 'Oblivion' - This ending unlocks Doyle's story and the extras section
  Don't answer it – Leads to ending 5 'Oblivious'

After viewing the 'Oblivion' end, you can access Doyle's version of the story by clicking the image to the left of the main menu. However, because I am such a noob at Unity things, I couldn't get the buttons to work as intended. To actually get the button for that and the extras menu to work, you have to go into one of the other menus first (load, gallery, or settings) back out of that, then the button will be clickable. The extras menu is accessed by clicking on the red number 9. The load time is also very long when starting this route or going to extras. I need to add some additional load screens cos right now it just makes it look like the game is frozen >.<

Doyle's Story

This one is a little more difficult to get a couple of the endings for, as this time around, when it switches back to Leah for the choices, there is a point counter going on in the background. This counter only impacts 1 set of endings – After Life A & B. For one, you will need to make sure that Doyle's anger level is 4 or above. For the other, you will need to make sure that Doyle's anger level is below 4.

Choice 01:
  Invite him in – Continues the story, avoids building up Doyle's anger level
  Let him leave – Adds 1 to Doyle's anger level

Choice 02:
  Invite him in (2nd time) – Continues the story
  Let him leave (2nd time) – Forces you down a path that leads to two endings

If you picked 'Let him leave' twice, you will end up at Aku's house.

Aku's house choice:
  Kiss him back – Leads to ending 6 'Out In The Cold'
  Shake him off – Leads to ending 7 'Our New Home'

If you invited Doyle in, the story continues.

Choice 03:
  Tell him to move – Adds 1 to Doyle's anger level
  Let him sit – Avoids building up Doyle's anger level

Choice 04:
  Kiss him - Avoids adding to Doyle's anger level + changes dialogue and keeps other options open later
  Slap him – Adds 1 to Doyle's anger level + Cuts the scene short + changes dialogue + prevents you from getting ending 8 + locks you out of playing sexy scrabble with Doyle later and some of the other choices

Choice 05: – If you picked 'Kiss him'
  Stop him – Adds 1 to Doyle's anger level + Cuts the scene short and changes dialogue + prevents you from getting ending 8 + locks you out of playing sexy scrabble with Doyle later and some of the other choices
  Allow him to continue – Avoids adding to Doyle's anger level + Continues the scene and changes dialogue

Choice 06: – If you picked 'Kiss him' then 'Allow him to continue'
  Agree to let him stay – Leads to ending 8 'My Angel' (Doyle's PoV version)
  Decline his offer – Continues the story

Choice 07:
  Hug him – Avoids building Doyle's anger level + changes dialogue
  Do nothing – Adds 1 to Doyle's anger level + changes dialogue

Choice 08: – If you chose to kiss Doyle earlier on
  Tell him to back off – Adds 1 to Doyle's anger level + Cuts the scene short and changes dialogue
  Encourage him – Avoids building Doyle's anger level + Continues the scene and avoids Leah storming off

Choice 09:
  Tease him – Avoids the potential slightly NSFW scene
  Protest – Leads to the potential slightly NSFW scene

Choice 10: – If you chose 'Protest'
  Do as commanded – Only changes dialogue
  Question him – Only changes dialogue

Choice 11: – If you chose 'Protest'
  How about no! - Adds 1 to Doyle's anger level + avoids the slightly NSFW scene
  Bring it! - Avoids adding to Doyle's anger level + proceeds to the slightly NSFW scene

Choice 12:
  Kill Doyle – Leads to ending 9 'Lingering Love' + opens up choices for more endings
  End it all – Leads to endings 10 & 11 depending on Doyles anger level

If Doyle's anger level is 4 or above – Leads to ending 10 'After Life (A)'
If Doyle's anger level is below 4 – Leads to ending 11 'After Life (B)'

Choice 13: – If you picked 'Kill Doyle'
  Kill Doyle (2nd time) – Leads to ending 12 'All Fall Down'
  End it all (2nd time) – Leads to another choice

Choice 14: – If you picked 'End it all' (2nd time)
  Please leave – Leads to ending 13 'Letting Go (A)'
  Please stay – Leads to ending 14 'Letting Go (B)'

Aaaand that should be it! If you get stuck on anything else though or have any other questions, please don't hesitate to drop me a comment or an email and I'll do my best to help :3