Lenneth Valkyrie is introduced as a recently awakened Valkyrie who was summoned to Valhalla by Odin, the supreme ruler of the gods. He orders her to explore the downtrodden world of Midgard in search for suitable human souls to become her Einherjar; to fight alongside the Aesir in the upcoming battle of Ragnarok.
At the beginning, Lenneth is accompanied by the goddess Freya as she finds her first Einherjar, but soon leaves her on her own. Throughout the game, Lenneth's personality is shown to be that of a stern, but good-natured woman. Along the way, she wins many battles and recruits various souls to her cause. However, her true story and past begins to unfold when she meets Lucian; a brave but insecure young warrior. They first laid eyes upon one another in Gerabellum when Lenneth visits the town in her human disguise; calling herself Meril . When they first meet, Lucian acts completely shocked and dumbfounded at her semblance to his first love; a girl with long-silver hair. While his girlfriend Claire grows in jealousy
Lenneth leaves the town, but later comes back to retrieve Lucian's soul after he loses his life protecting Claire. He continually seems dazed at the Battle Maiden's resemblance to Platina, his childhood friend who tragically died. Lenneth disregards this as a coincidence, but kisses Lucian before he leaves for Valhalla. She laments that there is no way for a human and god to be together. However she hopes for his survival in the coming war against the Vanir and looks forward to seeing him again.
Lenneth's journey in Midgard continues on, until one fateful incident. As Lucian continues to brood over his conflicting feelings for the Battle Maiden, Frei and Loki approach him. Lucian confides in them about his suspicions about Lenneth and Platina being the same person. Frei reaffirms his suspicions, stating that when a Valkyrie's duties are completed, she is put into divine sleep and allowed to live as a mortal for a time. Lucian is astonished to hear this, but wonders if he can get Lenneth to remember her past life as Platina and her love for him. Loki seizes the opportunity and tricks Lucian into using the Water Mirror to contact Lenneth. Lucian sends Lenneth an earring that once belonged to Platina and asks her to find the other pair. To his dismay, Lenneth views this as a breach of her trust and proclaims what he has done is unforgivable.
This crushes Lucian, while Loki shows his true self. He reveals that he has stolen the Dragon Orb, one of the Four Great Treasures, and plans to pin the blame on Lucian. The scorned warrior attempts to stop him, but his strength is no match for Loki. Lucian is easily defeated in combat. Lenneth is speechless when Freya informs her of this incident.
Then, Lenneth returns to the Weeping Lily Meadow, where Platina died. There, she finds an earring. This causes an intense reaction in her mind, as she realizes it was the other earring Lucian asked her to find. The seal that was placed on her memories shatters, as she witnesses her past life with Lucian.
Meanwhile in Valhalla, Odin and Freya, concerned with the seal being broken, decides to replace Lenneth with her older sister; Hrist. Hrist sports a much darker side than Lenneth as she willingly takes over Lenneth's body without any qualms. Arngrim and Mystina, Lenneth's Einherjar and most trusted companions refuse to serve Hrist when she ordered them to obey her. In their defiance Hrist prepares to kill them but her energy blast was intercepted by Lenneth, who sacrifices her soul to protect them.
Arngrim and Mystina panic as they try to figure out how to bring Lenneth back. Lezard Valeth, the psychotic necromancer who lusts after the silver-haired Battle Maiden, appears and instructs Mystina on how to go about restoring her soul. It is crystallized and transferred to a half-elf homunculus who bears an uncanny resemblance to Platina. However, the three of them still needed to retrieve the Valkyrie's true vessel, so they decided to storm Brahms' lair and defeat Hrist. Brahms lends them his strength. During this battle, Lenneth's frantic voice can be heard, begging Arngrim and Mystina not to die. After Hrist was defeated, Lezard and Mystina crystallize her vessel and used it as a catalyst to revive Lenneth. Although her revival was successful, she suffers an intense emotional breakdown; hysterically sobbing over Lucian's death. After pulling herself together, she takes her Einherjar to Valhalla to confront Loki.
In Valhalla, Lenneth speaks with Freya, who snaps at her for giving in to her human emotions. Lenneth now sees the arrogance of the Aesir, and claims that she no longer cares since she now desires to protect the humans. Once she makes it to Loki, she draws her blade and challenges him. However, she is unable to stop him from destroying Asgard and Midgard with the Dragon Orb. As Lenneth grieves over the destruction, the cries of the people awaken Lenneth's new inner power. The fact that she was reborn as a half-elf homunculus has given her this ability to grow in power. This causes her to attain the power of creation. She restores the two worlds and destroys Loki with the aid of her companions and her new sword; Glancer Reviver.
At the end of the game, Lucian is reborn. He and Lenneth embrace each other and now they can live together in peace.