Kain Highwind was lord captain of the Dragoons and a good friend to Cecil in Final Fantasy IV. Kain later became controled by the wizard Golbez. However, over time, he would overcome all the brainwashing attempts and became a true ally of Cecil.
Final Fantasy IV
Kain is the son of Richard Highwind, Baron's Dragoon commander, who died when Kain was young. He was then adopted and raised by the King of Baron. He became friends with Cecil Harvey, the King's son, but often fought each other due to Kain being jealous at the King's favored treatment to Cecil. They also befriended a priestess named Rosa Farrel, who usually broke their fighting. Kain loved Rosa, but both Rosa and Cecil were attracted to each other, setting Kain as the rival in both friendship and love sense with Cecil.
The King of Baron eventually wanted Kain to become the Dark Knight like Cecil, but to honor his father, Kain chose to become a Dragoon.
As the game begins, Kain was sent to Mist Village along with Cecil to run an errand, killing a Mist Dragon along the way. When they arrived at the village, the errand turned out to be a Bomb Ring, burning everyone inside. This event horrified Kain to realize that there's something wrong with the king himself, but in the same time, they found a little girl, Rydia, crying over her dead mother, in which they realized that because she's a summoner, the two just killed her mother by killing the Mist Dragon. After deciding not to follow the king's orders anymore, Cecil and Kain tried to take Rydia along with them, but she, still hating them for killing her mother, lashed out by summoning Titan, separating the duo together.
Kain was not seen again, but during Cecil's defense of Castle Fabul's Crystal, he reappeared and completely antagonistic towards Cecil, defeating him in combat and would've killed him if Rosa hadn't stopped him. The new commander of the Red Wings, Golbez, arrived, possibly having manipulated and brainwashed Kain into his servitude, captured Rosa and claimed the Crystal.
Eventually, Kain was stationed in the Tower of Zot and guard Rosa. Despite his request for a rematch with Cecil, Golbez refused the request and sent an Arch Fiend to deal with Cecil, but the fiend was defeated. Later, Kain offers to use Rosa as a hostage to deliver the Earth Crystal, and Cecil went through the deal. On the Earth Crystal's delivery, Golbez was planning to turn against the deal. However, the mage Tellah attacked him with the spell Meteor at cost of his life, which broke Golbez' control over Kain. Apologizing at both Cecil and Rosa and admitting that he was brainwashed, Kain rejoins Cecil's group, revealing the location of the Dark Crystal on the underground.
He continued to assist Cecil, but on the trip on the final Crystal, Kain was once again brainwashed from afar by Golbez and snatched the Crystal away. He was not seen again until Golbez himself was freed from the control of the Lunarian Zemus. Once again on his senses, Kain rejoins the party and offers that he be killed in instant in case he gets brainwashed again. Together with Cecil's group, Kain ventured through the Lunar Subterraneae and defeated Zemus and Zeromus.
When everything's over, Kain did not attend to Cecil and Rosa's wedding, burdened with guilt over his jealousy. He decided to train in the Mount Ordeals to become the perfect Dragoon.
Final Fantasy IV: The After Years
Kain was seen in the sequel game, once again becoming villainous and aiding the Mysterious Girl. However, hot on his trails was a mysterious hooded man. As the game proceeded, it turned out that the hooded man was Kain himself, and the villainous Kain was his escaped dark side from his trial in Mount Ordeals. After defeating his dark side, Kain is promoted into a Holy Dragoon, and aids Cecil's son Ceodore, and later on returned to Baron for good.
Dissidia Final Fantasy: Duodecim
Kain was a warrior at Cosmos' side during the 12th conflict of Dissidia. Over time, however, he learned that there was a sudden increase of Manikins that would permanently kill the warriors if they defeat them, with Cosmos unable to revive them. Conspirating with the Warrior of Light, Kain proceeded to secretly knock out and defeat the Warriors of Cosmos, betting that they would be victorious in the 13th Circle instead. Their plan was eventually found out by Lightning, and eventually, she and the remaining warriors, Laguna Loire, Tifa Lockheart, Vaan and Yuna agreed to go with their plan to go into the Empyreal Paradox and prevent more Manikins to enter the realm. Kain knew this was a suicide mission, but he has no regrets, and joined them to fight the Manikins. Although he was slain and unable to be revived, he succeeded the mission, and in the 13th Circle, the threat of the Manikin in a horde was prevented, enabling the Warriors of Cosmos to claim victory.