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Activate, discard a card from your hand -> Return target character card from your KO'd pile to your hand.

Activate, discard a Brotherhood character card from your hand -> Return two target character cards from you KO'd pile to your hand.
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If there is only one Brotherhood character card in my KO pile and nothing but Brotherhood character cards in my hand, can I choose to use the first activated ability rather than the second?

The real question is does the card's power trigger off of what I announce or what I pay with?

yes u can choose to only take one card from your ko'd pile, when u activate the power u choose your targets at the same time, so u cannot discard a card and then pick that card back up.

You announce which effect you wish to use.

The answer is yes, if you can pay for the first but not the second effect, you can still elect to use the first effect.