Hello! Don't worry, some of these endings are hidden pretty well. Here is a guide to help you get them:
For Xiao (Endings 9 and 11):
Ending 9: First, play Kazuha's April Fools' route and refuse to listen to his jokes to get the ending "No time for jokes". After getting that, go to Xiao's main story and choose not to read the messages.
Ending 11: This ending is actually hidden in Lumine's route! You need to follow the same path as Lumine's 10th ending: in the kiosk mini-game, you must collect the maximum number of black "..." bubbles (they appear on the left side of the screen).
For Wanderer (Dottore Encounters):
To meet Dottore, you must play Wanderer's Main Story on Hard Mode. You also need to have unlocked Xiao's "Monster...?" ending (the one where he kills Aether) beforehand. Then, when Wanderer takes you to the laboratory, you need to find Dottore's bug in the search mini-game.
For Flins (Ending 5):
In the hallway, when the main character suddenly stops by the photo, just wait out the timer (or choose the option "Something weird is going on..."). Later, choose to go for a walk in the evening and approach the strange grave on the right path.
For Tartaglia (Main Story Ending 3):
To get ending 3 in the main story, you need to complain about absolutely everything at the very beginning of the fishing trip! Just keep picking the bottom options (complain about the rod, the mosquitoes, the bait) collect every single complaint.
For Tartaglia (Maslenitsa Endings 2, 3, 4, 5):
To avoid falling into Dain's trap and still get to the cooking part, when Ajax asks who is texting you, choose the option about the Class rep.
After that, the endings depend entirely on your pancake cooking skills in the mini-game:
Ending 2: Hit the yellow area perfectly 4 to 5 times.
Endings 3 and 4: Make all the pancakes incorrectly (fail them).
Ending 5: Make exactly 2 or 3 pancakes correctly.
Good luck getting 100%!