Can my spell Not So Fast counter the Rogue Assassin's ability?

Question

Accepted Answer

If Rogue Assassin's ability actually "chooses a friendly unit or gear" (i.e., it creates a targeting/choice of a friendly unit or friendly gear when played or finalized), you may play Not So Fast as a Reaction to counter that ability. If the ability does not make such a choice (or the choice doesn't exist yet), Not So Fast cannot target it.

Yes — provided the Rogue Assassin ability is an enemy spell or ability that chooses a friendly unit or gear, you may play Not So Fast as a Reaction to counter that ability. If the ability does not choose a friendly unit or gear (or the choice hasn't been made yet), Not So Fast cannot be used to counter it. A Countered ability does nothing and is removed from the chain; costs are not refunded.

The opponent activates Rogue Assassin's ability; that ability becomes a Chain Item (Pending/Finalized depending on timing) and will be processed by FEPR. [337] When the ability is on the chain and it has "chosen" (created a choice/targeting relationship for) a friendly unit or friendly gear, you may play the Reaction Not So Fast (the card's text allows playing it "any time, even before spells and abilities resolve"). [337] Playing Not So Fast performs a Counter action targeting that ability on the chain. Countering removes the ability from the chain and makes it do nothing. Costs already paid to play the ability are not refunded. [425] If the thing chosen by the ability changed to a non-board zone before the Counter was played, the choice may be severed and the targeting relationship lost per target rules; whether the ability still "chooses" that object depends on that object's zone history. [359]

If Rogue Assassin's ability does not choose/target a friendly unit or gear (for example it targets an enemy unit, or it is a passive effect that does not make a choice), then Not So Fast cannot target it. If the ability only makes its choice during resolution (i.e., the choice is not made when the ability is finalized but happens as it resolves), then you cannot play Not So Fast earlier because the condition "that chooses a friendly unit or gear" is not yet true. You must wait until the choice exists (which typically happens during Finalize). [337] If the chosen object is moved to a Non-Board Zone (banished, hand, etc.) before you attempt to counter, the chosen object is no longer the same object and the ability may no longer "choose" it — this can prevent Not So Fast from being applicable. [359] If you successfully counter the ability, it does nothing and is removed from the chain; any costs the ability's controller paid remain spent. [425.1.a] [425.1.c]

What matters for Not So Fast is whether the enemy ability "chooses a friendly unit or gear." The card text of Not So Fast allows you to play it as a Reaction to "Counter an enemy spell or ability that chooses a friendly unit or gear." Under the Counter rule, when you Counter a card or ability it does nothing and is cleared from the chain, and costs already paid are not refunded [425].

Mechanically, opponents' abilities become items on the chain and are processed through the FEPR/Finalize process; choices and targets for those items are usually made during Finalize [337]. If Rogue Assassin's ability, when played/finalized, creates (chooses) a friendly unit or friendly gear as part of its play, that condition is satisfied and you may respond with Not So Fast and Counter that ability [425]. If the ability does not make such a choice (or the choice is only determined later during resolution), Not So Fast cannot be used to counter it until/if that choice exists. Additionally, if the chosen object moves to a non-board zone before you attempt to counter, the targeting relationship may be severed and the ability might no longer be considered to choose that object [359].

Because the exact text of Rogue Assassin's ability was not provided, I cannot state definitively whether it "chooses a friendly unit or gear." If you can provide the ability text, I can give a definitive ruling. Absent that, the most defensible interpretation is: yes if it chooses a friendly unit/gear; no if it does not or if the choice hasn't occurred yet.