From the new spoiler of Valar Morghulis found here: http://new.fantasyflightgames.com/edge_foros_discusion.asp?efid=16&efcid=4&efidt=651350

11. Maester Kerwin --- Character --- Greyjoy
Cost: 1
STR: 1
Icon: I
Trait: Maester
Crest: Learned

RESPONSE: Kill Maester Kerwin (cannot be saved) to cancel any effect that chooses 1 of your Ironborn characters as its only target.

Question: Can I use Maester Kerwin's ability to cancel a player playing an attachment on my Ironborn character in the marshalling phase (not by Ambush, the normal way with gold)? As far as I understand, this is neither a card effect nor a triggered effect. Is marshalling a card considered an effect at all and thus cancellable by Maester Kerwin?

Ferozstein said:

The keyword in this cancel effect is "target". This mean the effect must have you "choose" the character in order for it to be considered a "target". Attaching an attachment on a character is never choosing it as a target.

It's worth noting, however, that you are correct in that playing a card is considered a player action, not an effect of any kind.

can he cancel passive ability?

I don't see anything in his text which says it has to be triggered.

Making him one of the broadest cancels available (there are cancels for multi-targeted or non-targeted triggered effects, but only one that works on passive effects - and that one is very specific), probably why it's pretty much a one-shot.

Khudzlin said:

That one, and Outwit.

Kerwin is more generic than Outwit, because Outwit can only cancel "When revealed" plot effects (not other passive effects or triggered effects). Not to mention that he can stay on the table for more than 1 round.

Hmm. He can cancel a "when revealed" if the plot effect "chooses" an Ironborn character as its only target? Sounds like it to me. Now I really think he's cool. He can cancel Search and Detain!

Well, your opponent is likely to choose another card (assuming he wins initiative and chooses you as the first player), like a location or Kerwin himself.

Khudzlin said:

For the most part, that is a win for you over them choosing that expensive Ironborn character. Like Victarian or Euron Crow's Eye!