Osa

Osa is Leader of"Shinigami no ichizoku(Clan of Reapers)

Manga
He had been lived his castle called "Shinigami-jou( Castle of Reapers) with his clans.He and his clan members used to be hated by other peoples.4yers ago, Kokujo stole some apples and chased by shop keeper.He run into Shinigami-jou and met with Osa. At first, Kokujo frightened because of Osa's appearance and rumors,but Osa force Kokujo in his Castle.After he took Kokujo in his castle, he gave an Apple and  fair clothes to Kokujo, beacause he was wearing only tattered trousers and he was terribly thin.

Osa ordered Kokujo to take care of pigeons, hoping it might be good for Kokujo's mental.

Time was passed,Kokujo found stature of Dolballom. Osa told Kokujo He is the Keeper of Darkness Civilization card,and he was hated by all of people due to those Darkness cards.When Kokujo knows Osa's secret, Kokujo told him He will keep them with Osa.Osa was surprised and joyed by those words.He gave Kokujo Darkness deck and taught him how to duel.

After one year, Kokujo was becoming strongest duelist in the clan. Osa took Kokujo to Duel Masters World Tournament.When they arrived arena,Kokujo noticed there was a lot of bodies and Zakira. Kokujo was frighten and run into his castle but he saw his castle and clan members was destroyed.When Osa arrived castle after Kokujo,he told he betrayed them for moneys and He had hated all of Darkness Civilization and his clan, but he also tried to introduce Kokujo to Zakira for keep him from those destruction. Kokujo shocked by those words and challenged him to duel. Osa accept this but he declared he would never use Darkness cards anymore.He tried to destroy Kokujo with his new Lightness deck.But Osa was destroyed by Kokujo's Darkness deck and his Dolballom after all.

Chage
His appearance was only few seccond in Charge.

Cross
He appeared  in Episode36.

Deck
He might use Darkness deck when he met Kokujo. But after he betrayed his clan, he used Lightness deck.

Trivia

 * Osa(長) means "leader" in Japanese.
 * His true name is unknown.