A “defending character” is any character that is participating in the challenge on the opponent’s side. Characters the defending player controls that are not participating in the challenge are not defending characters.
A character is considered to have a challenge icon or to not have that icon. A single character that has and/or is gaining the same challenge icon from multiple sources functions as if it has one instance of that icon. (See Challenge Icons.)
Since Sandstone does not require you to “choose” a character, opponents will need to decide whether to use a cancel or not before they know which character will be gaining power.