If I have Master Yi legend, and one of my units is defending alone, but during the showdown I play a Rengar in reaction, does my unit still have +2?

No, your unit loses the +2 might bonus the moment Rengar, Trophy Hunter enters the battlefield.

According to RiftJudge FAQ #3935, the bonus provided by Master Yi, Wuju Bladesman is a continuous effect that relies on a "while" condition ("While a friendly unit defends alone"). This condition is checked constantly; the moment another unit enters the same battlefield, your original unit is no longer "alone" (as defined by Rule 741.1), and the bonus is removed immediately.

Interaction Details:

1. Passive vs. Triggered: Master Yi's bonus is a continuous passive ability, not a triggered ability. It does not use the chain and does not require "triggering."
2. State Check: Because it is a continuous effect, it updates the game state instantaneously whenever the criteria change.
3. Result: When you play Rengar as a reaction, the game state updates as soon as he enters the battlefield. Your unit is no longer defending alone, so it immediately loses the +2 might bonus.

This interpretation is based on the continuous nature of Master Yi's ability as confirmed in RiftJudge FAQ #3935 and FAQ #9025.