Use Z to select, and X to go back. Using kind ACTs will increase your "mercy level", increase it enough and you can spare the monster. Alternatively, you could kill it. In Heart Test mode, click the little icons to select them.
--THIS PROJECT IS UNFINISHED AND VERY GLITCHY-- I never really got around to finishing this, and it's super buggy, AND it's not very accurate to the original battle system. Still, I put a lot of effort into this, so I couldn't let it go to waste. "THE KING'S REVENGE" IS VERY INTENSIVE ON WEB BROWSERS, PLEASE USE THE OFFLINE EDITOR FOR IT —————NOTES and CREDITS————— Feel free to use this, but it's pretty overcomplicated. Credit to Toby Fox for almost everything.