The Demon King Seems to Have Infiltrated the Hero's Party, Vol. 1 (light novel)

Kyle was reincarnated into a new world as a member of the hero’s party. He’s forged unbreakable bonds with his crew over the course of their journey—Noelle the High Priestess, Dal the thief, Tiger the beastfolk, Sasha the mage, and Eclair the hero. Their expedition appears to be going well, but before long, a goddess appears before Kyle and tells him an earth-shattering truth: one of his dear friends is actually the Demon King in disguise. Can he figure out the identity of his greatest foe ...