However, I cannot afford to have you waste all my efforts with the power of light that you hold... Once you are gone, eternal life will be mine!
Xande is the third student of the great Archmage Noah. When Noah died, he gifted his third pupil, Xande, with the gift of dying naturally. Upset about receiving such a gift, Xande eventually went to the Syrcus Tower, dispatched Kraken to attack the Water Crystal and then used the Earth Crystal to trigger the Great Earthquake. With two of the crystals sealed, he sends minions to the Floating Continent to take care of the remaining crystals while he summons more darkness.
However, his minions are foiled by the Warriors of the Light. The four youths defeat the mooks and eventually make it to the Syrcus Tower where, after a last-ditch attempt at feeding them to wyverns is foiled by the friends they made along the way, the Warriors confront and defeat Xande. However, as the mage lays dying, Xande reveals that the darkness has become too powerful and that the Cloud of Darkness draws near, preparing to return the world to the Void...
Final Fantasy is a series full of great villains. Xande, unfortunately, is not one of them. Most Final Fantasy games give a a central force that the party fights for a significant portion of it: FF1 has the heroes fight against the Four Fiends to restore the crystals. FF2 pits the heroes against the Palamecia Empire. FF4 eventually introduces Golbez for most of the game, but even prior it is clear that the kingdom of Baron is going in a dark direction. FF5 begins with the heroes trying to figure out why the crystals are shattering before the other 2/3rds focus on ExDeath. The first half of FF6 focuses on fighting against the Empire while the second half has Kefka rise to godhood.
Xande is something of a prototype of the latter three villains, being out of focus for the first half of the game only to be revealed as the true instigator later. However, Xande is only defined during the final fourth of the game when the party meets Doga. Prior to that, Xande is just a name mentioned by Medusa and Kraken. Unlike the Emperor and the Empire, there's a significant amount of monsters that have no clear tie to Xande, hurting his presence compared to the former. Likewise, the only time Xande is seen in person is when the party fights him; All of the other mentioned villains including Garland in technicality appear multiple times throughout the game. Xande doesn't get that.
To put it into perspective, Xande is Zemus, but if he was fought in the Giant of Babil and died and Zeromus came out of nowhere and the heroes had to go directly to the moon afterward.
The remake makes some changes to Xande in order to improve the cohesion of the plot although, in true Xande fashion, most of these are either offscreen or indirect.
The major difference comes from the major changes in the storyline: Xande's motive is very clearly rooted in regaining the immortality he lost as a result of his mentor's "gift". To do this, he upsets the balance of light and dark to usher in an eternal night, freezing time. Another notable aspect is Xande's impact on the world: In the remake, Xande is the one to create the Floating Continent rather than the ancients. Combined with the time stop effects of the Flood of Darkness and it is implied that the Floating Continent has been cut off from the surface for 1,000 years. This has a few knock-on effects: Most notably, the protagonists new backstories mean that them being orphans, as well Desch being cryogenically frozen are now direct results of Xande's actions. This gives him a direct connection to the protagonists, although the game itself doesn't capitalize on it much.
The game also portrays Xande in more of a tragic light. In particular, Unei speaks favorably of Xande, especially in the unused dialogue. Perhaps in awareness of people feeling Xande was at least justified in his response to his gift, Luneth admits that he can't blame Xande for reacting the way he did. Doga and Unei do explain that it comes down to making the most of your time, giving a bit more of a reason as to why Xande's reaction is seen as "incorrect". Finally, the Warriors of the Dark clarify that Xande had no intention of summoning the Cloud of Darkness, only desiring to stop time as a means of avoiding dying.
Outside of his magic casting animation in the remake, Xande has no direct unused content. However, the Ultimania Archive reveals two interesting things about Xande: First, he was originally female (a "cold and beautiful woman.") which would have made him the first female Final Fantasy main villain, beating Ultimecia to the punch by several years. Second, Xande was originally slated to have his own boss theme; In the final game, he uses the standard boss theme, leaving the Cloud of Darkness with the only unique battle theme.