When Rengar Trophy Hunter ambushes a battlefield that you don't control, are you attacker or defender?

When you use Rengar, Trophy Hunter to enter a contested battlefield that you do not control, you become the Defender.

According to the rules for contested battlefields (Rule 442.1.a) and the provided RiftJudge FAQs (specifically FAQ #9992 and #9820), the player who first moved a unit to an unoccupied battlefield is designated as the Attacker. When you play Rengar, Trophy Hunter to that battlefield as a [Reaction], you are designated as the Defender.

As noted in the retrieved context, Rengar, Trophy Hunter's ability to be played to a battlefield without your units is a specific, unique permission provided by his card text, which allows him to function as a reaction even when you have no other presence there. Once he enters the battlefield, you are assigned the Defender designation.