A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Can i use Eihiko's reaction upon announcing Souhiko's action, reducing the cost of the personality while bringing him in play from my province?
Battle: If Souhiko is opposed: Bring a target face-up Personality from one of your provinces into play at the current battlefield, paying 2 less Gold.
Reaction: After you announce an action: Reduce the Gold Cost of a target card (in play) by 2 until the action ends.
Can i use Eihiko's reaction upon announcing Souhiko's action, reducing the cost of the personality while bringing him in play from my province?
Battle: If Souhiko is opposed: Bring a target face-up Personality from one of your provinces into play at the current battlefield, paying 2 less Gold.
Reaction: After you announce an action: Reduce the Gold Cost of a target card (in play) by 2 until the action ends.
Bolding by me.
You may not for the reason that the card Souhiko will target is not in play yet, as he is in your province.