All of If's endings are listed and detailed below, including their unlock conditions.
Ending 1
Collect all 23 effects and leave Shimitsuki's room.
After walking through a long hallway and opening another door, Shimitsuki will suddenly awaken all the way back to the first bedroom. She moves the wardrobe to hop back into her dream world once more as the credits roll.
Ending 2
Collect all 23 effects. Go back to Isolated House and interact with the window to the north. Go south in the room with effect eggs and interact with the egg in the middle. Select the first option on the prompt to drop off all effects. Go back up north and interact with Osoro to obtain the Name effect. Select it from the menu to forcefully wake up and change Shimitsuki's name to Izumi. Leave Izumi's room.
After walking through a long hallway, Izumi will see a mutilated torso of herself in a puddle of blood with a chaser standing in front of it. The chaser walks away from it, phasing through Izumi as they exit the room. The credits roll as Izumi walks towards the torso.
Ending 3
Collect all 23 effects. Go back to Isolated House and interact with the window to the north. Go south in the room with effect eggs and interact with the egg in the middle. Select the first option on the prompt to drop off all effects. Go back up north and interact with Osoro to obtain the Name effect. Select it from the menu to forcefully wake up and change Shimitsuki's name to Izumi. Instead of leaving Izumi's room, go back to sleep and go back once again to Isolated House.
Upon reaching the house, instead of Isolated House, Izumi will be taken to a long hallway with portraits of characters hanging on the wall. Izumi changes into each of her 23 effects briefly as she walks. The typing of keys can be heard in the distance. Once the player reaches the end, Izumi sees a tall Osoro at Izumi's laptop, typing. The credits roll as one final tap of a key is heard, and Izumi is suddenly nowhere to be found.