In the original game, Sephiroth is a fairly generic evil bad guy whose mystique is entirely built on basically being a cryptid. He's mentioned as existing once or twice in Midgar before killing the president offscreen. Cloud tells Barrett about Nibeliheim because Barrett asks who this "Sephiroth" dude people keep mentioning is, and you spent pretty much the entire first half of the game trying to find him.
In Remake, he's hinted as being in the first reactor watching Cloud, appears to Cloud in a dream outside the reactor, appears to Cloud in another dream in the church, appears to Cloud in another another dream in the VR machine, shows up at Shinra Headquarters and talks with them a bit, stabs Barrett, and then has a 45 minute multi-phase final boss fight against the whole party in a dimension beyond time before doing some sultry foreshadowing and piecing out.
Despite this massively increased role, when you leave Midgar Barrett still does not actually know who Sephiroth is. No one ever actually tells him for the entire first game, and he presumably spends the entire 45-minute final boss fight in the dimension across time wondering why this goth twink time lord stabbed him.