Sephiroth is the main antagonist of Final Fantasy VII, and one of the major villains of its extended universe. A prototype SOLDIER and the strongest alive, Sephiroth is initially revered for his strength and hailed as a hero, until he learns and misinterprets his origins. Believing himself as the last of the Ancients, Sephiroth enacts a vendetta against mankind, with the ultimate goal of becoming a god and controlling the planet. Sephiroth's goals later include defeating Cloud Strife, who has consistently foiled his plans. To this end, Sephiroth is frequently depicted as Cloud's archnemesis.

Sephiroth is arguably the most popular villain of the Final Fantasy series. Sephiroth, in all his appearances, wears a long black coat with silver pauldrons, black boots, and black clothing underneath. The top of his clothing underneath his uniform is split to reveal his chest, covered by crossing leather straps. His hair is long and silver, a sharp contrast to Cloud Strife's blond spikes, with his bangs parted to either side of his face. Beyond his clothing and hair, Sephiroth is known for his green cat-like eyes and the Masamune, his seven-foot long katana that he is rarely seen without. Sephiroth's typical battle stance with the sword is to hold it over his left shoulder, and he usually wields the Masamune two-handed, presumably to increase the force of his strikes as he is capable of using it with one hand. In later appearances he has a single black wing emerging from his right shoulder.

Sephiroth's personality is one of cold ruthlessness. Prior to his madness, he was not particularly anti-social, as he has friends in Crisis Core Final Fantasy VII and willingly tells Zack and Tifa about Mako and Materia on Mt. Nibel. All the same, his cold exterior and professional attitude turn people away. He is very intelligent, and is respectful to those he deems worthy - he considered Professor Gast a great scientist, but looked down on Professor Hojo and described him as "a walking mass of complexes". Sephiroth is confident to the point of arrogance, but as the strongest SOLDIER alive it could easily be argued he has the right to be. Well-spoken and somewhat graceful, Sephiroth is universally calm, collected, and in control. He can be frustrated and caught off his guard, but this rarely happens.

Besides Final Fantasy VII, Before Crisis, Crisis Core, and Advent Children, he also appears in Dissidia Final Fantasy on the side of Chaos and Kingdom Hearts series as an optional boss.