If I respond to an opponent taking an open Battlefield with Rengar Trophy Hunter, am I attacking or defending?

According to RiftJudge FAQ #9992 and FAQ #10056, when you respond to an opponent moving a unit to an unoccupied battlefield by playing Rengar, Trophy Hunter, you are the Defender.

Here is the breakdown of the designations:

1. Attacker: Your opponent (the player who first moved their unit to the open battlefield). According to Rule 442.1.a.1, the player who first causes a battlefield to become Contested is designated as the Attacker.
2. Defender: You (the player who plays Rengar, Trophy Hunter to that same battlefield as a [Reaction]). According to Rule 442.1.a.2, by playing a unit into a battlefield already contested by an opponent, you become the Defender.

This interpretation is based on the rules for Contested status (Rule 445/446) and the definitions of Attacker/Defender designations (Rule 442.1.a).