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, 13 March 2008
Mirumoto Kei (Experienced) Battle: Kei challenges a target Unique Personality opposing her. Other players can not take Reactions while this action is being targeted and resolved. Destroy the loser.
Swift Counterattack Reaction: Before a player resolves a Battle action that was taken as an additional action: Negate the action's effects.
Doomed Intentions Battle: If there are enemy units at the current battlefield: Move one of your target units there. If it moved, straighten it. If you are the Defender, take an additional Battle action printed on a card in the unit.
Determination Dark Virtue Reaction: Before you announce an action, target one of your Samurai: The action can not have its effects negated or have its targeting changed. After the action destroys any Personality, remove him from the game.
So, my opponent plays Doomed Intentions and brings Mirumoto Kei XP into battle and plays her ability, and I play Determination + Swift Counterattack to negate her ability since it's the second consecutive battle.
Is Kei´s battle negated by Swift Counterattack+Determination, or not ?
Thanks.
A player, 13 March 2008
Assuming your opponent is the Defender and he uses Kei's Battle ability as the additional Battle action granted by his Doomed Intentions, then yes: your Swift Counterattack negates Kei's action's effects and you needn't play Determination to do so.
Indeed you are forbidden to take Reactions while Kei's action is being targeted and resolved but your Swift Counterattack is played before this Battle action resolves, which means between the 'Pay Costs' step and the 'Action Resolves' step, as written in rules about timing:
Responses happening after or before a period of time (such as a turn or phase) do not count as part of that period of time.
As a consequence your Swift Counterattack is played neither while Kei's action is being targeted nor while it is being resolved, so this Reaction is legally taken.
However take note that Determination never allows you to ignore card texts such as "Other players can not take Reactions" because they are not negation effects, they are restrictions on the legality of taking Reactions, as explained in the glossary about "Cannot" (emphasis added):
EXCEPTION: In card text, when referring to entering play, targeting, attaching, using a trait, or taking an action, "cannot" and "can only" are restrictions on the legality of that thing in the first place, not negation of its effects.