When a unit moves to a battlefield during an ongoing showdown (after the initial unit that started the showdown moved there), who is the attacker and who is the defender?

Ruling: When a unit moves to a battlefield where a showdown is already in progress (the battlefield is already contested), the newly arriving unit becomes the defender, not the attacker. The unit that originally moved to the occupied battlefield and applied the "contested" status is the attacker.

Nuances:

• This is different from moving to a battlefield after a showdown has already ended and the battlefield is conquered - in that case, the newly arriving unit would be the attacker
• This interaction is important for units with "when I attack" effects like Leona, as those effects would not trigger if the unit is moved in as a defender during an ongoing showdown