Solved! Thanks to Mr. Q. for pushing my thoughts in the right direction.
(EDITED to present this in a more
Umineko-ish fashion:
Spoiler for Battler fails at chess:
Battler stared at the board. The whole game seemed to be totally illegal - it started out soundly, but then the black king just jumped across, like, five squares.
"Of course he did! Are you surprised, Battler? This is the power of the witch! I had Gaap open a portal to move him aaaallll the way over there."
There was a pause as Battler considered this feat of magic and/or cheating, and then he firmly facepalmed. "You know," he said, "I feel really damn stupid now. I get it. Here's the trick I have to use..."
After he performed the 'trick' in question, the game was entirely legal. "This looks familiar," said Battler. "I think Kanon did this to me once. It's an old trick."
"Indeed! And they even named it after you, Battleeeer!"
Battler groaned at the witch's mad laughter. Well, at least he knew the key now.
Thanks to Jan-Poo for an excellent puzzle and a great game.