I don't think you can remove the target in response no? You would have to remove a creature before the decision on what is being copied right?
The Supplanter would attack and then its effect would go on the stack and you would have to blindly kill the best card to copy before the effect resolves I would think, since it seems you immediately Exile the target and copy it in one move at resolution.
Though I'm no rules expert and without the actual rulings on the card out yet I guess we have to wait and see how it functions.
Wait, Supplanter of Identity has hexproof, meaning you can't target Supplanter of Identity instead of removing the creature it's going to copy? *gasp*
No, there's no hexproof mentioned anywhere?
The others where saying remove the target such as Bellower before you exile it, leaving an attacking 0/3.
However I was saying that I don't think you chose the target straight away since Supplanter would attack and put its effect on the stack then at resolution you immediately choose your target and immediately exile and copy it without a chance to remove the target being copied in response.
That would mean you would instead have to Blindly remove Bellower because it is the best creature to copy on the board but then Supplanter just copies something else instead.
That's how I read the ability anyway going off my limited ruling knowledge:
1. Supplanter Attacks
2. Supplanter's trigger goes on the stack but no choices are made yet.
3. Opponent removes Bellower because it's the best thing to copy.
4. Supplanter's trigger resolves and you choose a target and immediately exile and copy it, perhaps Whirler Rogue instead now because Bellower is in the grave.
The hexproof comment might be sarcasm. Maybe.
Anywho... the sequence is this...
1, Supplanter, and anyone else, declared attacking.
2, All "when this attacks" effects go on the stack, choosing targets as appropriate. (This is why it will be too late to benefit after copying a "when this attacks" like Ulamog.)
3, opponent removes the Supplanter target.
4, resolving the stack, Supplanter's ability has no target left. Normally this wouldn't matter, but the key here is that it says "if you do". That clause alters the outcome of the ability that's on the stack, so nothing happens.