Akuma/guide

Vespa
Enter the Light World. Go roughly south-west and interact with the scooter.

Machete
Enter the Neon World. Go south-east and enter the door. In the Neon Labyrinth, go up once, right twice then down. Interact with the dark tile. Make your way through the teleport maze. You should eventually reach a glitched platform with a red cube.

Interact with the red cube. Go up and interact with the spinning diamond. Go up a bit and right, interact with the machete (avoid the chasers).

Saxophone / Saxofón
Enter the Neon World. Go south-east and enter the door. In the Neon Labyrinth, go up three times then left twice. Interact with the saxophone.

Corruption / Corrupcíon
Enter the Darkness World. Go south-west and interact with the moving dark blob.

Star / Estrella
Enter the Mirror World. Go down the following stairs: Right, Down, Left, Down, Right, Bottom Right. Go left and up and interact with the NPC.

Scientist / Científico
This effect is only gotten through the debug menu. You can enter the debug room by interacting with the shining object on Akise's bookshelf.