I play against Yi. I go in with Diana on a BF with one Yi. I use Moonfall to bring Akshan. Does the Yi have +2 Might from defending alone at the beginning of Showdown?

To answer your question: Yes, at the very beginning of the showdown, Master Yi's defending unit will have the +2 Might bonus, but it will lose that bonus as soon as Akshan enters the battlefield.

Here is the breakdown of why this happens based on the game rules and established FAQs:

1. Timing of the Showdown

• Showdown Initiation: When your Diana, Lunari moves to the battlefield with the enemy Master Yi (or the unit being buffed by the Wuju Bladesman, Starter legend), the showdown begins.
• Passive Application: The +2 Might bonus from Wuju Bladesman, Starter is a passive ability with a "While" condition ("While a friendly unit defends alone"). As confirmed in RiftJudge FAQ #10141 and FAQ #10556, this is a continuous effect that updates the game state instantaneously.
• Initial State: At the moment the showdown begins, if the defending unit is currently alone on the battlefield, the game state recognizes the condition as met, and the +2 Might is applied immediately.

2. Interaction with Moonfall and Akshan

• Playing Moonfall: You play Moonfall as a reaction (or action, depending on the state) to move Akshan, Mischievous to that battlefield.
• Entering the Battlefield: Moonfall resolves, and Akshan moves to the battlefield.
• State Update: The moment Akshan enters the battlefield, your opponent's defending unit is no longer "defending alone." The continuous "While" condition of Wuju Bladesman is no longer met.
• Immediate Loss of Bonus: Per RiftJudge FAQ #10141, the game state updates instantaneously. The defending unit immediately loses the +2 Might bonus the moment Akshan arrives.

Summary of the Sequence

1. Showdown Begins: The battlefield is Contested. Master Yi's passive checks the board, sees the unit is alone, and grants +2 Might.
2. Chain: You play Moonfall. It goes on the chain.
3. Resolution: Moonfall resolves. Akshan enters the battlefield.
4. Instant Update: The board state changes (there are now two units). The Wuju Bladesman passive bonus immediately ceases to apply to the defender.

This interpretation is based on the continuous nature of passive abilities as defined in the RiftJudge FAQs #10141, #3935, and #10556.