If I attack an empty battlefield no one controls, opponent moves a unit into there during showdown. Am I attacking or defending now?

According to FAQ #4665 and Rule 459.2.b, you are the attacker and your opponent is the defender.

Here is the breakdown of how this interaction works:

1. Initial Move: When you move your unit to an empty, uncontrolled battlefield, you apply "Contested" status to it. This initiates an "Open Showdown" (as per Rule 446.1 and FAQ #6170). Because there is no opposing unit present, you are not yet in Combat, and you do not gain the "Attacker" designation yet.
2. Opponent's Move: When your opponent uses a spell or effect to move their unit into that same battlefield, they trigger what is known as "surprise defense" (Patch Notes - 2025-10-24).
3. Combat Transition: Because units from opposing players are now present at the same battlefield, the ongoing Showdown transitions into a Combat Showdown (Rule 447.1).
4. Designations: Per Rule 459.2.b, the designations are established as follows:
   • Attacker: You are the attacker because your unit was the one that first applied the "Contested" status to the battlefield.
   • Defender: Your opponent is the defender because they moved in second and did not apply the initial "Contested" status.

Important Note: As per FAQ #4665, the first open showdown must complete its resolution (if any chain items were played) before the combat showdown begins, but because the battlefield is now contested by two players, the first showdown will not result in a conquest. Immediately following that, the combat showdown begins with you as the attacker and your opponent as the defender.