Zephyros

Zephyros is the main antagonist of the game. He is mentioned throughout the game starting after you meet Kaeris, though you only hear that a dark power is attacking her village, and you don't actually encounter him until the Black Citadel. While at the Black Citadel he will kill Kaeris.

Background
Zephyros is the last of his people, who according to him, were hunted and killed for their horns. Centuries passed between Zephyros' people dying and his reawakening and corruption of the Mana Tree. It is interesting to note that the reported genocide of Zephyros' people appear to coincide with the beginning of the long peace in Evolandia.

First Encounter
You will first battle Zephyros at the Black Citadel. Unfortunately for you, you cannot win this battle. So just Attack to your hearts content and watch as the scene unfolds. He has 200 hp during his first phase, after which he will begin unleashing highly-damaging Kamehameha attacks. Even if you are able to overcome these and continue attacking, he will spontaneously heal himself for 500 hp at will, making him impossible to defeat.

He then kills Kaeris, and Clink unlocks the ability to summon Babamut. Doing so will win the fight immediately.

Second Battle
When the battle starts, Zephyros will only have one attack. It will slam it's hands down, potentially trapping you between them, and then breathes fire. To get past this stage, start running right and as it begins to track you, run left. Zephyros will continue turning to the right then slam its hands down, giving you a chance to attack the gems on its hand. Keep this pattern up until the gem is destroyed, then do the same for the other hand.

Zephyros now gets a new attack. He will lean towards the platform, and breathe fire while turning around so that a complete circle is made. Avoid the fire by running in the same direction until the boss stops moving. When still, attack the gem on his chest once, then run around the circle to target the orb that popped out from his body. Attack the orb as much as you can, and prepare for the next stage of the battle.

When you have successfully hit the orb a few times, Zephyros's armor will fall off and he will begin to attack with fireballs. Dodge these as best as you can by running. When he fires the blue fireball, face the center of the area, and swing your sword at the ball. This will send the ball back to the boss and will return the damage to him. After doing this a few times, Zephyros will fall, and you have defeated the game’s final boss!

Trivia
Zephyros is likely a reference to the god of the west wind in Greek Mythology, Zephyrus.

It is also a reference to the main antagonist in Final Fantasy VII, Sephiroth.

It is likely that him killing Kaeris is a reference to Sephiroth killing Aerith in Final Fantasy VII.

His last lines are a reference to Roy's monologue at the end of the 1982 film Blade Runner.