It was one thing to hear about one of your best friends having been a horrible monster back home, and another entirely to see it with your own eyes. The horrible mockery dripping from the Reaper's voice sends shivers down Frisk's spine, and the cruelty alight in his eyes just looks wrong to them. They gulp, repressing the urge to flee and praying for their Kuja, the real Kuja, to take them out of here. Even though they knew somewhere that the two were one and the same.