I have the Master Yi, Wuju Bladesman legend. The opponent moves a unit into an empty battlefield. I play Rengar, Trophy Hunter to that battlefield as a reaction. Is the Rengar considered a defender who would then benefit from Master Yi's "When I defend" trigger?

To answer your question, we must look at how the Defender designation is assigned and how Master Yi functions as a legend.

1. Is Rengar considered a Defender?

Yes. When your opponent moves a unit into an empty battlefield, that battlefield becomes Contested. According to Rule 442.1.a.1, the player who moved their unit there is designated as the Attacker. When you play Rengar, Trophy Hunter to that same battlefield as a [Reaction], you are placing a unit into a contested battlefield controlled by an enemy. By doing so, you are assigned the Defender designation (Rule 442.1.a.2).

2. Does Rengar benefit from Master Yi's "When I defend" trigger?

No. Based on the rules and retrieved FAQs regarding Master Yi, his "defending" ability is not a triggered ability—it is a passive ability (as confirmed by RiftJudge FAQ #9025 and FAQ #3743).

• Triggered vs. Passive: Because Master Yi's ability is passive, it does not use the chain and cannot "trigger" in the way an attack or defend trigger does. Instead, it applies as a continuous effect as long as its condition—defending alone—is met.
• Application: Once Rengar is the only unit you have at the battlefield, Master Yi's passive effect will apply to Rengar automatically and immediately. Rengar does not need a "When I defend" trigger to gain the benefits of Master Yi's legend ability.

Summary

• Designation: By playing Rengar to the contested battlefield, you are designated as the Defender.
• Legend Interaction: You do not trigger a "When I defend" effect because Master Yi's ability is a passive continuous effect. As long as Rengar is the only unit you have at that battlefield, he will receive the buff provided by your Master Yi legend automatically.

This interpretation is based on the definitions of Attacker/Defender designations (Rule 442) and the mechanical distinction between passive and triggered abilities confirmed in RiftJudge FAQs #9025 and #3743. No official FAQ exists for this specific combination of Rengar and Master Yi, but the rules for their respective ability types are definitive.