The Mirror Moment is the instant a player first sees their costumed self — in a mirror, on camera, or through someone else’s eyes — and realizes “I’ve really become this character.”
It is the most observable part of Second Self. The change in appearance builds up bit by bit while dressing, but the sense of “I’ve become it” lands at the moment of seeing.
Looking in the mirror is a step that dressing rarely skips, and it does two jobs: checking that the head sits right and the look matches what was intended, and confirming that one is ready. Both usually happen in the same glance. Once confirmed, the character state carries on without further looks; skip the step, and the wearer tends to feel uneasy.
The moment can happen alone, with no audience and no compliments. Someone else’s eyes bring a different kind of seeing: being taken for the character by another person.
Kigurumi makes the moment especially clear. Cosplay keeps the performer’s own face; kigurumi replaces the face as well, so nothing of the original appearance is left in the mirror.