Council of Elrond

When an agent is revealed, can the opponent respond to that action by playing "withdrawn to mordor"? and if yes, can the controller of the agent respond by playing "pilfer anything..."?

If revealing of agent is active condition of other action (playing card requiring agent), then it takes place at declaration. Opponent may fizzle this action by playing Withdrawn to Mordor in response.
If revealing of agent is action by iself (i.e. player declares that agent will be revealed), then it takes place at resolution. Opponent may fizzle this action by playing identical agent as character (e.g. using We Have Come to Kill) or its manifestation in response.

but, if that agent is revealed by itself, then oponent responds playing withdrawn, can the agent's owner respond with pilfer? or isn't it considered revealed since it hasn't resolved the chain of effects yet?

Playable on a face-up agent. If the agent has a mind of 5 or less, it is discarded. If its mind is 6 or greater, return it to its owner's hand. Alternatively, an on-guard card is discarded.

If revealing the agent is an action, then you must wait for it to resolve before playing Withdrawn (until the action resolves, the agent is not considered "face-up", so Withdrawn cannot target the agent).

If revealing the agent is an active condition of another card, then it is considered revealed and you may play Withdrawn in response to the declaration of the other card.

At least, that's the way I see it.

cleared, thanks

marcos wrote:but, if that agent is revealed by itself, then oponent responds playing withdrawn, can the agent's owner respond with pilfer?

If Withdrawn to Mordor was played on untapped revealed agent, opponent can respond with Pilfer Anything Unwatched.

I fully agree with Bandobras Took's post.