Rokka: Braves of the Six Flowers

When the Majin awoke from the depths of darkness, the deity of fate chose six heroes and bequeathed them with the power to save the world. Adlet, a boy who proclaims himself the strongest on Earth, was selected among the Rokka no Yūsha (Braves of the Six Flowers), and he goes to the rendezvous point — but seven have gathered there. The heroes suspect that someone among the seven is the enemy, and the initial suspicion falls on Adlet.

Synopsis from Anime News Network