Gecko Moria is a former pirate who's known best for fighting evenly with Kaido, one of the Yonkou (Four Kings). It is believed that he was eventually overtaken by Kaido in the New World losing everything he had in the process. With nothing to lose he now serves the World Government as one of the Royal Shichibukai though his true intentions are to reclaim his past glory and become the king of pirates.
Major Story Arcs
Thriller Bark Arc
In order to regain his prestige Moria enlisted the aid of the diabolical genius Dr. Hogback, the invisible beast man Absalom, and the ghost princess Perona. He then set sail on the Thriller Bark, a massive ship constructed out of a miniature island mass from West Blue. For years he would lure unsuspecting ships sailing through the Florian Triangle into his labyrinthine ship in order to steal the shadows of powerful fighters and implant them into the bodies of marios (patchwork corpses) created by Hogback. Eventually he created an army of zombies soldiers before being utterly defeated by the Straw Hat Pirates. After his defeat he managed to escape with the help of his underlings and lived to fight another day.
When the execution of Portgas D. Ace was announced Moria was called to Marineford by the World Government to help defeat the Whitebeard Pirates. He accepts the summons with the intent to rebuild his zombie army using the shadows and corpses of the world's strongest pirate crew. When the war finished Moria was stripped of his rank as Royal Shichibukai by Doflamingo and a squad of Pacifista who presumably acted on orders from the highest authority of the World Government, the Gorousei (Five Elders).
Powers and Abilities
The Kage-Kage fruit gives him control over shadows, enabling him to transplant another person's essence into anything he pleases, as well as manipulate his own shadow to do all the fighting for him.
Moria can physically touch a person's shadow giving it tangibility. This allows him to sever the shadow from the physical body of the original owner using his signature giant scissors. Anyone who loses their shadow immediately loses consciousness. If a person comes into contact with sunlight without a shadow their body will disintegrate.
Once removed a shadow's essence can be placed into another body either living or dead. Whatever body receives the essence gains the fighting ability of the original owner by mimicking the movement of the shadow. If put in a dead body the shadow becomes anchored in the body as long as it doesn't come into contact with the sea or its respective proxies (i.e. sea stone, salt). If placed into a living person they temporarily gain a boost in battle strength and skill that typically lasts for about 10 minutes. Moria uses this power to build a loyal army of zombie soldiers and to boost his own fighting strength if needed.
Moria also has direct control over his own shadow in the form of an entity known as "Doppleman." It fights with the full strength of his actual body with the added benefit of changing it's shape at will. The amorphous shadowy substance that forms Doppleman's body can be turned into a variety of shapes at the capacity of the fruit user's body mass. Forms often inherit animal-like shapes and typically include piercing spikes as a primary mode of attack. Attacking the shadow does no damage to Moria himself and he can instantly change places with his shadow body when attacked. The only exceptions to this rule are when Moria has absorbed shadows into his own body as well as when using the effect of "Kage Kakumei" (Shadow Revolution).
He can also directly implant Doppleman's shape changing powers into the shadow of a zombie body under his control. This creates the effect known as Shadow Revolution which causes the physical body to alter shape to reflect the shadow connected to it. Basically a zombie's physique will gain unnatural elasticity that Moria can mold or stretch into nearly any shape limited to the actual body's mass.
Implanting excessive quantities of stolen shadows into a living body causes a notable increase in muscle mass. Normal bodies can't handle the strain of this boost but certain Devil Fruit powers can adapt to the size change. As a last resort Moria can simultaneously absorb all shadows within range of his domain directly into his body with the ability known as Shadows' Asgard. He can grow massive in size and gains formidable amounts of strength depending on the exact number of shadows under his dominion. His mental awareness becomes lower but he can still deploy Doppleman to attack. The only way to reverse this transformation is to cause enough physical damage to Moria's body to essentially wring out the shadow essence.