Kaido

Kaido is one of the Four Emperors in One Piece. He is the one who killed Gekko Moriah's old crew. He also tried to attack Whitebeard but was stopped by Shanks. He is also considered to be the strongest creature of the world due to having an army of Beasts, and he himself is a beast. He hires Donquixote Doflamingo to make his army.

Personality
Not much is known about Kaido's personality but it is implied by Trafalgar Law that he has a temper. He would even kill Doflamingo if something happened to the SMILE factory.

Abilities
As a Yonko, Kaido is an extremely powerful individual. He is much stronger than Doflamingo, the latter who is incredibly strong by himself. He is even strong enough to fight Whitebeard (another powerful Yonko) from going to Marineford. Despite Kaido's menacing capabilities, not much is known about his actual powers; however, it is heavily hinted that he is a powerful Zoan devil fruit user.

Past
At some point, Kaido met Doflamingo and they decided that Doflamingo should supply artificial devil fruits to enhance Kaido's army. And at another point, Kaido slaughtered Gekko Moriah's comrades in the New World.

Summit War Saga
Before the Marineford War, Kaido attacked Whitebeard but Shanks stopped the fight.

Pirate Alliance Saga
Luffy and Law decided to create an alliance so that they can take down one of the Yonko, Kaido. With the SAD factory on Punk-Hazard destroyed, the Straw-Hats and Law now want to destroy Kaido's SMILE factory in Dressrosa so that it can lower Kaido's army power.

Trivia

 * Kaido is the only Yonko who have not made an actual appearance.
 * Many fans believe that Kaido could be a Mythical Zoan devil fruit user.