Xulicfer, Seeker of Death

Xulcifer is the ultimate being of Darkness. He was created in an alternate dimension when the Mysterious Figure (Young Xehanort) and Aqua fell in love. He quit the plan set in motion by his older self and helped Aqua defeat him, thus ending the Xehanort Saga even before the events of KH1. Then, they settled down and had a child. However, Xehanort's Darkness-tainted blood caused him to be born with the soul of a demon and an unending thirst for hearts. He absorbed both their hearts, instantly growing into an adult, and set off to the actual dimension of Kingdom Hearts to consume all the powerful hearts he sensed there.

Likes:

Black clothes

Consuming hearts

Converse shoes

Belts

Zippers

Fighting

Linkin Park

Dr. Pepper

Role in the story
Xulcifer seeks to consume all the hearts in the universe due to his demonic thirst. In KH3, he challenges Sora multiple times throughout the story, always going easy on him, as he likes to play with his prey. When Kairi is praying with the other Princesses of Heart in other to open the door to the sacred Realm of Holy Light, he falls down from the ceiling and stabs her with his Keyblade, Eternal Thirst, thus consuming her heart. However, consuming a heart filled with light countered his demonic nature, and something woke up within himself while he was taunting Sora, who was shocked over the loss of one of his best friends. Kairi's heart cried from inside, causing him to cry tears of Darkness. From there on, he can summon Kairi's Keyblade with his own Keychain and wants to consume Sora's heart more than anyone else's so he can stop crying tears of Darkness. It is rumored that Nomura will make him the main antagonist in the next saga, titled "The Death Seeker Saga".

Abilities
Xulcifer is incredibly powerful, growing stronger with every heart he consumes. The Darkness in him is so intense that he has the abilities of both Heartless and Nobodies, who answer to him over anyone else. He can call forth dark meteorites capable of destroying entire towns. In his world, he travelled to Olympus Colosseum and consumed the heart of every god there, so his power has achieved a level beyond divine. Once he gets Kairi's Keyblade, he can fuse it with his in order to create the Apocalyblade, a gigantic Keyblade that stands taller than the stratosphere. He is also incredibly beautiful and charming when he wants, as he preffers to steal girl's hearts figuratively before doing so literally.

He can command both the forces of Darkness and Light, and so, no element has any advantaged against him. However, he enjoys fighting more than consuming hearts, and thus he will restrain himself from using his full power when fighting an above-average opponent.