Throughout the different worlds, Mariana can find 12 total different alters in the game, each of which give her a different appearance. They each provide different effects and can be used to interact with the different worlds she comes across to varying degree.
Alters
Verdant
(Alters recommended to have before retrieving: none)
Appearance: Mariana becomes plant-like, gaining wood-like skin and green hair. She is decorated in vines and flowers, with a white sundress on.
Passive Effect: Plant.
Action: Bloom pink flowers all over yourself all Mariana to attract.
Location: Found in Old Factory through door opposite the one which leads outside to the Victorian Stairs. Climbing the middle ladder and entering will take you to the isolated balcony with a pink flower. Interacting with this pink flower will give the Verdant alter.
Practical Uses: Attracts NPCs. Needed to unlock Ending #3.
Damage
(Alters recommended to have before retrieving: none)
Appearance: Mariana becomes injured, with sickly white skin and light blonde hair. She dons a hospital gown, with bandages covering her head and right eye.
Passive Effect: Injured. Can cause NPCs to be wary of you.
Action: Allows Mariana to use her implanted claw to damage certain objects and kill NPCs.
Location: Found in Hospital deep in one of the rightmost rooms of the dark green corridor. Inside this room is medical equipment and a large experiment tank, containing a white skinned red-eyed creature. Interacting with this creature will give the Damage alter.
Practical Uses: This effect allows Mariana to kill NPCs in certain areas, and causes negative reactions.
Pastel
(Alters recommended to have before retrieving: none)
Appearance: Mariana's hair and clothes become pink/blue pastel colors.
Passive Effect: None.
Action: None.
Location: Found in the pastel house in Mauve Shoal. Go southwest until you see a castle on a heart-shaped island. Inside is a pastel house with three lamps at the back wall. Moving between the pink lamp or the blue lamp and the wall will take you to another room. Flip the light switch on the wall and interact with the girl sitting on the bed.
Practical Uses: Needed to get the Apparition alter.
Nostalgic
(Alters recommended to have before retrieving: none)
Appearance: Mariana has brown hair and wears an orange and brown outfit.
Passive Effect: None.
Action: None.
Location: Found in the Train Station. Use the stairs to either your left or your right and keep going until you reach another staircase, go up them and take the right stairs and go through the third set of double doors. Go down and take the small door to your left, then take the center double doors and go up the stairs. Interact with the girl on the swing.
Practical Uses: None.
Glassy
(Alters recommended to have before retrieving: none)
Appearance: Mariana wears a hat and dress that resemble stained glass.
Passive Effect: Extremely dark areas become brighter.
Action: None.
Location: Found in the Lamp Forest, go west and slightly south until you find a stained glass lamp and interact with it.
Practical Uses: Lights up some areas that are otherwise very difficult to see in.
Vortex
(Alters recommended to have before retrieving: none)
Appearance: Mariana's hair turns bright pink and she wears a purple dress. Her skin becomes solid black.
Passive Effect: None.
Action: (Holding space) A portal envelops Mariana and makes her seem to disappear.
Location: Found in Escher Garden, go left under the trees, then northwest under the trees again. Navigate your way around this area westward, then go down around a group of trees to a lightning-bolt-shaped waterfall. Go southeast until you find a huge pool, then take the path directly below the pool and go west where it branches. Keep going until the trees south of you thin out under the pagoda like roof or alcove/pavilion, then go south and take the winding to the small brick building. Interact with the NPC inside.
Practical Uses: This allows Mariana to teleport to the Nexus at will.
Gelatinous
(Alters recommended to have before retrieving: none)
Appearance: Mariana turns into blue jelly.
Passive Effect: Mariana bounces when she walks.
Action: None.
Location: Found in the Alien Ocean. This area has a lot of twists and turns and is difficult to navigate, but somewhere in the area is a narrow pathway surrounded by stone. At the end of the pathway is a small blue jellyfish. Interact with it to get the effect (this will also wake you up).
Practical Uses: None.
Monochromatic
(Alters recommended to have before retrieving: none)
Appearance: Mariana's hair and clothes become greyscale.
Passive Effect: None.
Action: None.
Location: Found in Black Lace, go slightly northwest until you see a monochromatic girl. Interact with her to get the effect.
Practical Uses: None.
Toxic
(Alters recommended to have before retrieving: none)
Appearance: Mariana becomes green and wears a hazmat suit.
Passive Effect: NPCs and Plague Nurses run away from her.
Action: None.
Location: Found in Neon Sewers. If you've already unlocked the shortcut from Rainbow Scratch Park to the Nexus, go northwest (and around the playground) until you see a manhole surrounded by a fence. Go down into the sewers and enter the only doorway. Take the only other entrance in the next room, then go right and through the doorway. Go left and up through the only other doorway in the room. Go up and keep doing right and take the rightmost doorway. At the end of the path is a green slime, interact with it to get the effect.
Practical Uses: Prevents chasers from catching you.
Celestial
(Alters recommended to have before retrieving: none)
Appearance: Mariana becomes light blue and she has wings, a halo, and a starry robe.
Passive Effect: None.
Action: None.
Location: Found in the Planetarium in Dark Town, keep travelling south until you get to a somewhat open area with a sane Plague Nurse wandering around. Take the boat to the right, and keep going right into a large waterway until you see a dock and stop the boat there. Take the two ladders up, and then the left ladder. Go left and through the double doors into the Planetarium, then into the only doorway that isn't roped off. Interact with the little star to get the effect.
Practical Uses: None.
Metallic
(Alters recommended to have before retrieving: none)
Appearance: Mariana turns into an android made of gray, black, and orange metal.
Passive Effect: None.
Action: None.
Location: Found in the Abandoned Lab, take the first path downward and left and enter the next door. Interact with the deactivated Plague Nurse in the fourth room from the left.
Practical Uses: Needed to unlock Ending #3.
Apparition
(Alters recommended to have before retrieving: Pastel)
Appearance: Mariana becomes semi-transparent, with blonde hair and a pink jumper skirt.
Passive Effect: None.
Action: None.
Location: Found in Misty Forest. Go down and left, then right, around the carousel and down once more, then right and down, then down again, take the lower left path, and keep going down until you reach the neighborhood. Go right and enter the door. Equip the Pastel alter in the last room on the upper floor and interact with the creature on the bed.
Practical Uses: Makes you invisible to chasers.