Villainous Pact Timing

Hi there!
I have a question about the timing of Villainous Pact and when the 'non-villain character' that is permitted to block must be selected. Does that selection occur when Villainous Pact is played or during the attack step?
For example, if my opponent has no active characters when I play villainous pact, but they are able to field a character via the Collector or Mister Sinister globals afterwards, is that sidekick die permitted to block in the subsequent attack step?
Thanks!
I have a question about the timing of Villainous Pact and when the 'non-villain character' that is permitted to block must be selected. Does that selection occur when Villainous Pact is played or during the attack step?
For example, if my opponent has no active characters when I play villainous pact, but they are able to field a character via the Collector or Mister Sinister globals afterwards, is that sidekick die permitted to block in the subsequent attack step?
Thanks!