Meyneth was once a human scientist working alongside Zanza (then called Klaus) on a space station orbiting Earth. She tried to stop Klaus from activating the experiment, as the results from previous tests were unconfirmed, but was literally pushed aside. When the Bionis and Mechonis are subsequently created, Meyneth created life in the form of Machina on Mechonis.
War of The Titans
When Zanza began his attack on Agniratha, Meyneth used her body as the Mechonis and retailiated by attacking the Bionis, and therefore Zanza. In the aftermath, both gods were exhausted and needed to rest to replenish their powers.
By some unknown chance, Vanea has Meyneth's soul, which she transfers to Fiora in a Mechon body many centuries later. While Meyneth and Fiora occupy the same body, neither are fully aware of each other's presence at first. Meyneth completely controls the body at first, although comes to share it for a while later.
Meyneth shows herself in her Faced Mechon at Prison Island and attempts to talk with Zanza using telepathy. However, Shulk attacks her, and her Face Mechon is damaged, revealing Fiora to be the pilot. As Meyneth does not have Fiora's awareness or her memories, she doesn't recognize Shulk, and retreats with the rest of the Mechon.
Meyneth attempts to talk with Shulk and the party later, when they reach the end of Valak Mountain, but she's stopped by Metal Face and brought back to Galahad Fortress by Egil. She is then remotely controlled to attack the party when they reach the fortress and try to rescue Fiora. After the party is defeated by Egil, Fiora regains consciousness and begs Meyneth to attack Egil, which causes the destruction of part of the fortress and the party and Face Nemesis to plummet to the Fallen Arm.
When Fiora awakes at Fallen Arm, it seems that Meyneth has gone into remission inside Fiora's mind. Fiora insists that they uncover the identity of her alter ego, in order to pay her back for her help at the fortress. Shulk reluctantly agrees.
After advancing onto Mechonis Field, Meyneth appears from time to time to aid the group and give advice. She shields the party from Gadolt's blast, and she makes him regain his heart and free will in Agniratha. She attempts to reason with Egil, but he rejects her, feeling betrayed for her siding with lifeforms of Bionis.
After Shulk defeats Egil and is about to make a truce with him, Dickson shoots Shulk through the chest, releasing Zanza. Fiora and Meyneth challenge Zanza, and attack him, however during the fight Zanza notes that Meyneth is not putting effort into the battle. He realizes that feelings for Shulk are the cause for this, and Zanza aims a blast of energy at him. Meyneth protects him, and Zanza fires a much stronger blast at the party; saying, "Face me like a god, or say goodbye to your putrid friends!"
Meyneth leaves Fiora's body to defend them, and is killed by the blast. Her last words decree that the party should strive to "create a world with no need for gods."
She disappears in a bright burst of light, and her Monado falls towards Egil, who reaches up to catch them before Zanza pulls them back, laughing. "This, is the passage of fate!" he cries, before teleporting away and reanimating the Bionis, attacking the Mechonis once more. Egil moves the Mechonis, fighting against Zanza, knowing his destruction is inevitable.
It is later revealed that Meyneth, locked in her own Monado, was Fiora's life force after her transformation. It is unclear how long Fiora would survive for after Meyneth was no longer supporting her.