Player A moves a unit to an empty battlefield (no units from either player). A showdown opens. During the showdown, Player B uses Ride the Wind to move one of their units to that same battlefield. When the showdown ends, who is the Attacker and who is the Defender?

According to the rules and FAQs for Riftbound, here is the ruling for your scenario:

Ruling: Player A is the Attacker, and Player B is the Defender.

Reasoning: The Attacker and Defender designations are determined by who first applied the "Contested" status to the battlefield. Because Player A moved their unit to an empty battlefield first, they applied the Contested status and are designated as the Attacker. Player B, by moving their unit to that same battlefield during the showdown, becomes the Defender.

Sequence of Events:

1. Player A moves a unit to an empty battlefield: This applies the Contested status to the battlefield and initiates a non-combat showdown. Player A is now the player who applied the Contested status.
2. Player B plays Ride the Wind: Player B moves their unit to the same battlefield during the ongoing showdown.
3. Showdown concludes: The non-combat showdown continues until both players pass in sequence.
4. Combat begins: Once the non-combat showdown resolves and both players have units present, a combat showdown begins.
5. Designations assigned: According to Rule 442.1.a.1 and 442.1.a.2, the player who applied the Contested status (Player A) is designated as the Attacker, and the other player (Player B) is designated as the Defender.

As noted in the FAQ: "The player who applied contested status to the battlefield first (by moving a unit there) is the attacker, regardless of when the other player's units arrive at the battlefield."