Sin is a Homunculus. He is the main antagonist in the Return of the Homunculi arc.
He was created when Ling used a philosopher's stone to turn the Emporer into a humunculous,to prove that a stone does grant immortality. This stone was father's failsafe plan. It was made with a bit of each humunculous' power. Sin has the power of each humunculous and the memories of Father. Upon turning he had Ling and Lan Fan taken away, as he decided to continue his rule. He then used 4 royal guards to make a new Greed,Lust,Pride,and Gluttony. He had special plans for making the last 3. (Will be revealed later.) He currently plots to use shing's forces to cause a world war that leave him ruler of it all. (His greed side makes him want this even more.)
Sin has the abilities of each homunculus.
Sin can manipulate, slice through or devour anything that comes in contact with his shadow and it appears invulnerable to conventional physical harm, although it can be deflected by sufficiently hard objects; Carbon-hardened objects appear particularly resistant. He can also see and talk through his shadowy eyes and mouths wherever his shadow can extend. This is supported by the shadows limited mobility, only being able to appear in the immediate surroundings of his human body. It also cannot appear without a light a source and can be harmed when light becomes too bright.Pride's shadow can consume living things and assimilate the abilities and strengths of that particular victim, along with their corresponding weaknesses.
Sin has the ability to shape-shift into anything or anyone, should it be real people or human aspects that it made up. He can transform to anything that doesn't exceed the size and weight of his monster form, which, seeing how massively large it is, does not create much of an issue. This power comes in handy to go anywhere unnoticed or to travel fast, allowing him to shape-shift into an average citizen to blend into a crowd for stealth, or a fast moving animal, such as a horse or dog. Additionally, Sin can transform parts of his body, often using this ability in battle to transform its limbs into weapons.Even in its preferred human form, Sin's appearance belies his actual weight, granting him enormous strength and durability. In spite of its immense weight, Sin is very fast in all forms it assumes. It is also noted that even in his human form he can turn his arms into extendable (to an unknown length, however), apparently sharp, tentacle like appendages.Under its monstrous form, Sin can use its strength to its maximum and becomes all the more dangerous; such as being easily able to crush rock under its limbs or jaws. It is also able to manipulate the human remains that cover its body and make them appear and disappear, and to elongate its tongue to use it like a harpoon or a binding rope, a continuation of its transformation ability.
A formidable opponent in battle, Sin is very deadly when using swords, which he wields with deadly proficiency thanks to his unique gifts and lifetime of training, to the point where he can wield five blades simultaneously. Already a supreme swordsman and martial artist, he is made even more dangerous by his Ultimate Eye. (concealed by his hair in the picture above) Though the details of his eye's abilities have not been elaborated on, it apparently grants him tremendously accurate vision and the ability to read and predict practically any move his opponent could make as well as the minute details and movements of his general surroundings. The eye's only weakness is that when there is a physical obstacle blocking its path, it is unable to provide Sin with the predictions it normally would. Additionally, he has pointed out on multiple occasions that, while his eye is capable of reading any situation, his own body is often unable to keep up with the projected courses of action that his sight ability proposes to him.
In combat, Sin uses his fists coupled with his astonishing physical strength. He is more than capable of crushing and tearing through solid rock and steel with his bare hands, and can easily crush a human body with accidental ease. Sin's hide is also extremely tough, rivaling his own carbon skin in durability, although it is much more flexible and seems to be a permanent feature of his body. Nothing short of tank or artillery fire could pierce it from a distance. In his most devastating attack, he hurls himself towards his foes at incredible speed. In fact, his speed is so formidable, he appears to have no means of controlling his momentum, and is thus incapable of stopping himself without crashing into something.
He has the ability to re-arrange the configuration of carbon atoms of his body, allowing him to convert his skin into Grapheme. He usually uses this ability to partially strengthen the density of selected parts of his body during battle, but can extend the effect to its entirety. In this form, he resembles a black-skinned demon with a malevolent facial visage. As Sin has control over the configuration of his body's carbon atoms, the substances that he can convert his skin into can vary based on his intentions. He can choose to convert his skin into the indestructible substance, or he can turn his skin into weak graphite. Why he would ever do this is unknown.
He can simply eat anything using his powerful jaws and acidic saliva. He also posses gluttony's ultimate sense,making very hard to hide from him.
He can extend his fingertips into long, flexible, thin blades sharp enough to cut through just about anything with ease. The length to which his fingers are able to stretch has not been shown to have any particular limit as well, earning them the nickname "Ultimate Spear".