Hiii thank you so much for playing the game !! I'm glad you enjoyed Yuki <3333
To get ending D, you have to make sure not to interact with anyone, not even the old couple !
The endings guide probably got you confused, it was made very early, and I realized only now that it's numbered, and in the old format ! I'm very sorry for that! I just noticed it. Lemme see if I can go edit it!