Can a non-turn player use a Reaction removal spell to kill an opponent's unit during the opponent's turn before they move to a Battlefield (outside of a Chain or Showdown)?

Ruling: As the non-turn player, you can only use Reactions when a Chain is open or during Showdown. You cannot use Reactions freely during the opponent's turn outside of these windows.

Sequence:

• If the unit has a "when I'm played" ability, that will open a Chain, and you can react to that trigger
• If the unit doesn't have such an ability, you must wait until a Chain is opened or a Showdown is in progress

Nuances:

• You cannot preemptively use Reaction removal before the opponent moves to a Battlefield unless one of the above conditions is met