Jack

Jack is a pirate captain who has ties with the Underworld.

Appearance
Not much is known about Jack, besides the fact that he wears some kind of metal jaw or helmet, bearing a slight resembelence to Morgan's or Jean Ango's. He also seems to have very bloodshot eyes.

Personality
Jack is very headstrong and reckless, deciding to attack four Marine ships which contained an Admiral, three Vice Admirals, and a former Fleet Admiral to rescue Doflamingo. Although, his own strength and that of his crew is unknown, so there is a chance it is somewhat feasible.

Kaido
Jack seems to be allied with Kaido, or even part of his crew, as his ship is elephant shaped, matching the animal theme of Kaido's Zoan army, and even bears his Jolly Roger.

Donquixote Doflamingo
Jack also appears to be allied with Doflamingo as he was seen planning to retrieve the former Shichibukai from his convoy.

Punk Hazard Arc
He was first introduced being notified of Caesar Clown's capture.

Dressrosa Arc
He appeared again tailing Marine Ships that were escorting Donquixote Doflamingo to Impel Down. Despite warnings that powerful Marines were guarding him, Jack refused to leave and prepared to attack the Marine ships.

Trivia

 * He shares his name with the nickname of a famous pirate, John Rackam, better known as Calico Jack.