Karst's main reason for being in Golden Sun: The Lost Age, which is the only reason she is ever mentioned, was to take revenge for her fallen sister, and light the lighthouse of Prox. She, just as her sister, Menardi has a companion along with her. Agatio, who is speculated to be Saturos's brother, accompanies her as well.
 In Golden Sun: The Lost Age
When Karst is first introduced, she appear in Madra after Felix and party return from trying to find a boat in the eastern part of the world. She questions Felix about where her sister is, and if she's alive. Well, obviously Felix, being the honest person he is, tells her that she's dead, and was killed by Isaac. Naturally, Karst gets infuriated and runs off to murder Isaac.
Later Felix and company confront Karst, and her now introduced partner Agatio, along with Alex in Champa when Felix tries to leave to city. We learn that Karst and Agatio were assigned the mission that Saturos and Menardi were given to ignite the Jupiter Lighthouse and Mars Lighthouse, and that they don't need Felix and his companions anymore. After a little campaigning, Karst and Agatio leave Champa.
Karst and Agatio and not seen again until Felix reaches the top of the Jupiter Lighthouse. After finding out that Mia and Garet have fallen down a trap in the lighthouse, Felix sees Issac and Ivan trying to help them back up. Karst and Agatio come out and tell them that they set the trap, and then attempt to fight the pair. Although Issac and Ivan put up a decent fight, Karst and Agatio ultimately win the fight.
Karst and Agatio deem Felix as a traitor and opt to kill him. The fight starts. During the fight with Felix, the team can either win or lose. If you win, you are rewarded normally.
Later in the game, you will meet Karst again. She and Agatio will have been transformed into the Fire Dragon's who will attack the party. The moves that the dragon's know are the same as they did before and the only thing that has changed is the HP and the stats. After the fight, Karst, along with Agatio will die, giving their last words to Felix.
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