Rokka: Braves of the Six Flowers(2012)

Rokka no Yuusha

When the world is threatened with destruction, six chosen heroes will rise to save it. One of them is Adlet Meyer--who calls himself 'the strongest man in the world.' But when he answers the call to assemble with the other heroes and face the darkness, there are not six heroes, but seven. Who is the traitor in their midst?

(Source: Yen Press)