Shadow: Until the end of the phase, characters defending this attack get -1 [Willpower] and are discarded if their [Willpower] is 0.

If I have a defending character with 1 willpower, is it discarded?

i do not know this card.. but from that rules paste.. YES... the card modifies the printed value of your will from 1->0.. then the effect says discard at zero...

yep, it would appear so, and of course the attack goes undefended