Jack

Jack is a Pirate Captain who has ties with the Underworld.

Appearance
Jack wears some kind of metal jaw or helmet and has a very small pupil.

Personality
Jack is very headstrong and reckless, deciding to attack four Marine ships which contained an Admiral, three Vice Admirals, and the former Fleet Admiral to rescue Doflamingo.

Kaido
Jack seems to be allied with Kaido, or even part of his crew, as his ship is elefant 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.

History
He was first introduced during the Punk Hazard Arc, being notified of Caesar Clown's capture. He appears during his planned attack on the Marine Ships escorting Donquixote Doflamingo to Impel Down.