The source of the ability is taken in to consideration for protection.
702.16b A permanent or player with protection can’t be targeted by spells with the stated quality and can’t be targeted by abilities from a source with the stated quality.
Since Diaochan is a human, a permanent with protection from Humans can't be targeted by her abilities. The same can be said about pro red creatures.
Situation: Activate Diaochan, choosing Knight Exemplar. Opponent says I cannot because Exemplar is Pro-Humans due to Riders of Gavony.
Question: Since Diaochan herself does not target Exemplar, is her activated ability still good?
Sub-Question: If creature is Pro-Red, would Diaochan's activated ability still be good?
You need to use the card's Oracle text:
Quote from Diaochan, Artful Beauty »
:Destroy target creature of your choice, then destroy target creature of an opponent's choice. Activate this ability only during your turn, before attackers are declared.
Since Diaochan's ability targets, a creature with protection from Humans or from red is not a valid target.
player 1 has Diaochan, Artful beauty and kills player 2's gizzly bears. Player 3 has a true-name nemesis set on player 1. Player 2 wants to kill the TNN with diaochan's ability. What happens? what if TNN was set on player 2?
player 1 has Diaochan, Artful beauty and kills player 2's gizzly bears. Player 3 has a true-name nemesis set on player 1. Player 2 wants to kill the TNN with diaochan's ability. What happens? what if TNN was set on player 2?
For protection, it doesn't matter who makes the target choice for the ability. Protection looks at the source of the ability - Diaochan.
So, if True Name Nemesis has protection from Player 1, no player can use Player 1's Diaochan ability to target the TNN. If TNN has protection from Player 2, any player (even P2) can use P1's Diaochan ability on it.
player 1 has Diaochan, Artful beauty and kills player 2's gizzly bears. Player 3 has a true-name nemesis set on player 1. Player 2 wants to kill the TNN with diaochan's ability. What happens? what if TNN was set on player 2?
If Nemesis is set to player 1, it can't be the target of Diaochan's ability, so player 2 cannot choose it as the target. What matters is the source of the ability; so for Nemesis, what matters is who controls the source of the ability. That the target is chosen by another player doesn't matter. So in the second case, player 2 can choose Nemesis as the target, since Nemesis can be the target of abilities from sources controlled by player 1.
Private Mod Note
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I'm a former judge (lapsed), who keeps up to date on rules and policy. Keep in mind that judges' answers aren't necessarily more valid than those of people who aren't judges; what matters is we can quote the rules to back up our answers. When in doubt, ask for such quotes.
Situation: Activate Diaochan, choosing Knight Exemplar. Opponent says I cannot because Exemplar is Pro-Humans due to Riders of Gavony.
Question: Since Diaochan herself does not target Exemplar, is her activated ability still good?
Sub-Question: If creature is Pro-Red, would Diaochan's activated ability still be good?
"Nothing tears up a group of friends faster than a session of Mario Party." ~ ForsakenM~
Extra Info:
EDH, Modern, Anything with Planechase
Cockatrice Name: ForsakenM
702.16b A permanent or player with protection can’t be targeted by spells with the stated quality and can’t be targeted by abilities from a source with the stated quality.
Since Diaochan is a human, a permanent with protection from Humans can't be targeted by her abilities. The same can be said about pro red creatures.
You need to use the card's Oracle text:
Since Diaochan's ability targets, a creature with protection from Humans or from red is not a valid target.
player 1 has Diaochan, Artful beauty and kills player 2's gizzly bears. Player 3 has a true-name nemesis set on player 1. Player 2 wants to kill the TNN with diaochan's ability. What happens? what if TNN was set on player 2?
For protection, it doesn't matter who makes the target choice for the ability. Protection looks at the source of the ability - Diaochan.
So, if True Name Nemesis has protection from Player 1, no player can use Player 1's Diaochan ability to target the TNN. If TNN has protection from Player 2, any player (even P2) can use P1's Diaochan ability on it.