Seiko

Seiko at first seems to be another strange character in this strange world, but then she's shown as actually having some kind of free will unlike the other NPCs. She even moves around the area and even to other areas entirely.

At first, she's in the first left building in Achroma staring at some Japanese writing on the chalkboard, but then when Limbo approaches, she's notably caught off-guard at this and instantly dashes off, ending up going to the smaller building up ahead, still on the left. She seems like she was waiting for Limbo to follow to then tell him that this world will hurt them and thus, they need to have it destroyed. After that interaction, the player could then backtrack to find her in the other realms.

In Prota, she's seen sitting in front of one of the many boxes in a house with something in her hands and when noticing she's been caught, she runs upstairs and claims you're following her to antagonize her and tells Limbo to leave before crossing her arms and looking away, ignoring him so he leaves.

In Trita, she's found in one of the previously locked rooms, seemingly caught off-guard when seeing Limbo followed her again. When approaching her after this, she tells Limbo to fade from existence as she then stabs him in the gut before running off. When Limbo reawakens, however, Ian is standing in front of him and the wound and blood is gone as if it didn't happen.

In Deutera, She's standing around by herself once again, but seems a bit taken aback when seeing Limbo again, asking "You didn't die?". At this point, you then get to chose to flip her off or take her mask. When flipping her off, she has nothing to say, but when trying to speak with her again, her final dialog is "Everything became blank and now I am nothing.", which may suggest "becoming blank/nothing" means losing yourself completely in this world, where you become an empty shell of what once was a person, being completely unfazed by everything. But, when taking her mask, Limbo discovers that while what normally would fatally injure someone doesn't work, taking the mask to expose their face actually kills them.

Seiko's SE is a quick "Uhh", which fits incredibly well with the first encounter.

Appearance
Seiko is depicted with a fair-to-medium skin tone, black shoulder-length hair with short bangs, a mask with two narrow oval eye holes with two grey lines striking through them (forming an upside-down 'V') and a line for the mouth that's straight at the bottom and slightly curved at the top while being thinner than Limbo's lip line. She's wearing a dark gray long-sleeved shirt with a black skirt and black dress shoe flats.

Trivia

 * Seiko was originally conceptualized around a year and a half before Lost's creation, originally being a (scrapped) image to promote CR's other series "Darkened Ashes" with her as an lowerclass demon.