If a player used a card to destroy my target Ninja personality, and i use night watchers reaction, does he still have to discard the card he tried to use to destroy my personality?
Night Watchers
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.
If a player used a card to destroy my target Ninja personality, and i use night watchers reaction, does he still have to discard the card he tried to use to destroy my personality?
Yes.
http://rules.l5r.com/CE_Card_types#Strategy_cards: "After the action ends, the player discards the Strategy card, even if the action did not resolve." Emphasis added.
On: Night Watchers