personally, i think that glimmer is actually cyrah endal. it's kinda crazy, yeah, but cyrah died in a light leaping accident [ gethen shone a secondary light path on cyrah while she was leaping ] so the gethen's light path could have carried more than half of cyrah and carried her to a neverseen hideout to be interrogated.

maybe that's why gethen's mind didn't break, because he didn't actually kill anyone. plus, the neverseen wouldn't risk any of their main member's mental stability without knowing for sure that they could handle it.

glimmer is also a flasher, like cyrah is.

in legacy, glimmer said something about a promise made by lady gisela to her, maybe it was to save prentice or a deal to not hurt wylie. glimmer could have been in a dungeon the whole time and promised to help lady gisela do whatever she wants. and in exchange for freedom, she would help keefe get a new ability.

and it could be contradicted that cyrah did die because wylie watched her fade, but if her second half at the location gethen's lightleaper was set to had more of her body on that side, the half with wylie would have faded and left half of cyrah still alive. it's a bit far-fetched, but let's say the elves have intense medication and physical training to help cyrah recover from nearly dying. and wylie was still young, so he wouldn't know how much of cyrah was still with him after the incident.

i don't remember if anyone else was with him, but i believe he was alone.

now for physical appearances, cyrah had red hair, but tonics to change appearances exist and glimmer could be continually drinking elixirs to maintain her black hair.

in unlocked, she had to go back to the neverseen warehouse to get a few 'supplies'. it was never specified what was inside, but considering the warehouse was burned down afterward, it must've been important. maybe family heirlooms or just sentimental things she kept with her at the hideout.

when wylie was interrogated by the neverseen, you may argue that cyrah would never have let that happen, but if she had interfered, both she and her son would have been hurt badly. blackmail could have been used, and we know the neverseen knows a lot of things about the endals—why not some dirt on cyrah herself?

and although glimmer is described as only a few years older than the twins, we know the elves don't age like humans and could look and sound young. plus, in the photo of prentice, cyrah, and wylie, she was described as young-looking.

also, it was brought up that cyrah's concentration was broken during light leaping, but if gethen and fintan had been there, why couldn't they have shone a secondary light path in her way?