Saga

Saga (サガ) is the "main antagonist" of the fifth movie. He's a childhood friend of Zoro's (both went to the same dojo together) and Maya's friend and possible love-interest. The real Saga is a friendly, honorable, and ambitious young man.

Abilities
Saga is a swordsman, but the loss of his right arm seems to have robbed him of his skills. However, when he used the Seven Stars Sword, it healed him of his defunct, and he gained a massive boost in strength, but falls under its evil influence.

History
He fell into a state of depression after losing the use of his right arm, but was brought out of it by Maya, whom he met when he drifted to her island. He was accepted into her village and later fought alongside them when they were attacked by pirates.

During the fight, he and Maya ran into the shrine that housed the Seven Stars Sword. He fell under the evil influence of the sword when he used it out of desperation to save her from the pirates. The sword possessed Saga to try to rise to its full power and bring darkness to the world. Saga took over and possessed an entire Marine dojo to help in the sword's quest.

When the Straw Hats landed on the island, Saga summons Zoro; at first to stop Maya from using the sacred jewels to seal the sword's power, but then plans to use Zoro as a live sacrifice to increase the sword's strength. Luffy manages to destroy the sword with the help of Maya's barrier, but the curse had already fused with Saga's body, giving him superhuman strength and allowing him use both of his arms again. Despite being weakened by Saga in an earlier duel, Zoro arrives and finally manages to free him of the influence with a one-sword strike. Saga survives and resolves to continue seeking the "sword of justice," his dream, to make up for his actions.

Trivia

 * Both Saga and Shanks lost an arm to save a friend at sea, Zoro and Luffy respectively, and both are still proficient swordsmen despite their handicap. Instead of dismemberment, Saga's was merely disabled. He regained use of his arm when the sword took over his body. It is unknown if he can still use his arm after the curse was lifted.