Impostor Syndrome

If the copied creature has {X} in its mana cost, X is 0.

Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any "as [this creature] enters" or "[this creature] enters with" abilities of the creature will also work.

Except for not being legendary, the token copies exactly what was printed on the original creature and nothing else (unless that permanent is itself copying something else; see below). It doesn't copy whether that permanent is tapped or untapped, whether it has any counters on it or Auras attached to it, or any non-copy effects that have changed its power, toughness, types, color, and so on.

If the copied creature is copying something else, then the token enters as whatever that creature copied, with the listed exceptions.

If something becomes a copy of the token, the copy also isn't legendary.