I am attacking an uncontested battlefield. My opponent plays Rengar the new orange one. He has Master Yi leader from Proving Grounds. Is it now a surprise defender with the +2 from Yi?

Yes, Rengar, Trophy Hunter will be designated as a Defender, and he will receive the +2 might bonus from your Wuju Bladesman, Starter legend (assuming he is the only friendly unit at that battlefield).

Here is the breakdown of why this happens:

1. Attacker and Defender Designations

• Attacker: When you moved your unit to the empty (unoccupied) battlefield, you initiated a non-combat showdown and that battlefield became Contested. According to Rule 442.1.a.1, you are the Attacker.
• Defender: Because Rengar, Trophy Hunter has a special ability allowing him to be played to a battlefield with enemy units (even if you have no other presence there), your opponent may play him there as a [Reaction] during that showdown. Once he joins that contested battlefield, your opponent is designated as the Defender (Rule 442.1.a.2).

2. Interaction with Master Yi Legend

• Passive Ability: The +2 might bonus from Wuju Bladesman, Starter is a continuous passive ability, not a triggered ability.
• Condition: The ability checks "While a friendly unit defends alone."
• Application: As soon as Rengar, Trophy Hunter enters the battlefield, he becomes the defender. Because there are no other friendly units at that battlefield, he is "defending alone." The game state updates immediately, and Rengar receives the +2 might bonus automatically.

This interpretation is based on Rule 442.1.a, the definitions of Attacker/Defender designations, and the mechanical distinction between passive and triggered abilities confirmed in RiftJudge FAQs #9877 and #10141.