Firefight Inconsistency.

[Jo and the Bush all/any]

In Steelheart, David figures out that Firefight is an illusionist due to a series of Pictures of Firefight, one of which Firefight disappeared because the illusionist lost concentration. But If Firefight isn't an illusion, is an actual Epic from a different universe, how does that work?

[Kobold King he/him]

I think it's because even though she isn't maintaining an illusion, she is consciously maintaining a window into the other universe. This probably takes concentration, which could be interrupted. Her interrupted concentration could be caught on camera, thus leading to David forming the faulty conclusion that she is an illusionist.

[Argent he/him]

Or the male!Firefight got out of range and Megan could no longer maintain her... interdimensional summon, for the lack of a better term.

[cemlub]

I think it's easier to think of as projecting a different dimension, than summoning another universe.  Because dimensions are infinite, what can be shown or done is similar to an illusionist.  However, it sounds like the closer / or more likely the dimension to her own, the easier it is to project.

[Dankworth he/him]

Maybe she flew him past a window that had a candle burning, thus activating her weakness and dispelling her projection.

 

Kobold King's probably right though; she most likely needs to concentrate to keep her extra-dimensional door/window/hologram thing running.