As a young soldier, Sephiroth is single-minded in pursuit of any goal given to him. He is conscious about every decision he makes, so approaches situations calmly and methodically. He has a strong sense of responsibility, and will humbly admit when he doesn't know something or makes a mistake. Despite the expectations placed on him, Sephiroth does not want to be a hero, instead dreaming of a normal life. Sephiroth maintains a pragmatic and slightly pessimistic outlook, but with the help of his comrades Glenn Lodbrok, Matt Winsord, and Lucia Lin he learns not to push himself to his breaking point constantly and allow himself to have fun.

Before his madness, as shown in Crisis Core -Final Fantasy VII-, Sephiroth maintains an outwardly aloof and professional demeanor during his time at Shinra. He is known for hardly mingling with anyone besides his only friends, Angeal Hewley and Genesis Rhapsodos, and being indifferent toward his fans. He can act humane and caring in contrast to his reputation, befriending Zack over their shared reluctance to hunt down their friends and desire to bring them back, and asking Cloud how he felt to be back in his hometown, having no similar experience himself.[note 2] Sephiroth is intelligent and respects those he deems worthy—he considers Professor Gast a great scientist but scorns Professor Hojo. Well-spoken and graceful, Sephiroth is calm, collected, and in control. He has a dry sense of humor. Sometimes, he can be frustrated and caught off-guard. As the strongest SOLDIER, Sephiroth is confident to the point of arrogance, yet has no interest in personal glory, as he tells Genesis, who envies Sephiroth's reputation, that he can have it.

After learning of the experiments that led to his birth, Sephiroth descends into madness, becoming murderous, ruthless, and vengeful, yet retaining his calm and cold demeanor. He develops a messiah complex, proclaiming to be "the chosen one" destined to lead the planet. He becomes sadistic and enjoys mentally torturing Cloud. Sephiroth is devoted to Jenova and her cause, even though her body serves as little more than his avatar. He has become an agent carrying out Jenova's will.[11]

In the Japanese releases, Sephiroth uses the pronoun ore prior to his insanity, a typical masculine pronoun used by confident males. Following the Nibelheim Incident, he begins to use watashi, a more formal pronoun that is genderless in formal context, likely because he now sees himself as superior to everyone else. This post-madness mannerism carries into his spin-off appearances, such as Dissidia Final Fantasy. In the Remake, Sephiroth uses ore at the ending, as he states he will not let himself end. All other interactions have him use watashi.

In the Final Fantasy VII Remake, Sephiroth still torments Cloud, gloating over his painful memories and failures. However, at the same time he encourages him to get stronger and survive his ordeals. He even beckons Cloud to join him and his cause to "save the planet" and implies he wants to alter his destiny. Rather than killing Cloud when he refuses, Sephiroth spares him and gives him a hint, letting him continue his journey.