Hashimoto Aoi manga
"A long time ago--there was a valiant swordsman who came to be known simply as 'the hero'. The people had been suffering at the hands of an evil demon. upon arriving, the hero challenged the demon to a battle...the hero and his companions combined their powers and brought the battle to a swift conclusion. The demon had been defeated! Thanks to them, the despicable monsters who were once controlled by the demon were freed from the demon's evil volitions. With no one around to cause trouble, the island became a quiet place where everyone could live together in peace." On a remote island in the southern seas lives Dai, a shipwrecked boy who dreams of becoming a great hero. One day, the evil demon king is revived, so it's up to Dai and his friends to stop him and the evil force that revived him. Along the way, Dai discovers the identity of "the hero", the truth behind the evil force who revived the demon king, and Dai's own hidden powers that surface in times of peril.
It's the Century of Darkness where monsters known as Vandels rule the world, and the Vandel Busters are the only ones who can fight them. When a group of Busters are killed in battle, they pass on their magic Saiga weapons to Beet, making him the strongest Buster ever. But Beet won't just fight the Vandels--he wants to destroy them and finally end the Century of Darkness.