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.
A player, 28 July 2007
Mercy Reaction: Play during the resolution phase of any combat that you have won. The opposing army is not destroyed, and you gain 3 Honor for each card spared. The Defender's Province may still be destroyed normally. (Errata: Attacker has the option of sparing the defending Province.)
Can I play this card as the attacker ally in a multiplayer game in order to win the honor myself instead of the real attacker player?
or is the attacker leader the one that wins the honor?
Now question with ninube ogoku.
Ninube Ogoku Unaligned Ninja Followers must be Ninja. Ogoku can join any army, including an army attacking your Provinces, regardless of whether you have been invited in that battle. You never gain Honor when Ogoku joins an army. Force 2 Chi 2 Honor - Gold 5
Can I use the reaction to avoid getting my own personalities get destroyed? or is the attacker who chooses?
Thanks for your time.
Zen Faulkes (rules official), 28 July 2007
Mercy: Can I play this card as the attacker ally in a multiplayer game in order to win the honor myself instead of the real attacker player?
No. Rules, Battle : "The winning army’s leader (the Attacker or Defender) is the winner of the battle."
Ninube Ogoku: Can I use the reaction to avoid getting my own personalities get destroyed?
I don't understand the question. Which probably means "No," whatever it is you're trying to do. Also, Rules Archive: "Her presence in an army will not change your overall role (Attacker/Defender/ally) in the attack. You also are not considered an ally if you assign her where you are not invited."