Leif was born in the time between Quan and Ethlyn's departure from Sigurd's army and their deaths at the hands of King Travant of Thracia in the Aed Massacre. After the fall of Leonster, he managed to flee alongside his guardian, Finn, and Nanna. During his childhood, he hid in several cities on the Thracia Peninsula, including Ulster and Tahra, but had to flee from all of them when the Grannvale Empire's soldiers came in search of him. Eventually, he and Nanna arrived in Fiana, where they asked the village's leader, Eyvel, to help Finn, who was wounded. Although Nanna begged in tears, Leif only glared at Eyvel and offered to give the Light Brand, a keepsake from Ethlyn, to her if she would help Finn.

At the beginning of Thracia 776, Leif departs on a mission to rescue Nanna and Mareeta after their capture at the hands of Raydrik and the imperial army. In Chapter 3, he and Eyvel surrender to Raydrik after he uses Nanna as a hostage against them. While Eyvel is taken to Munster's arena to meet with Nanna and Mareeta, Leif is imprisoned in the dungeon, where he meets Fergus and Karin. With help from them as well as the Magi Squad, he escapes the dungeon and travels to the arena. There, he rescues Nanna and witnesses Bishop Veld use his dark magic to turn Eyvel to stone. As a result of this, Leif begins working to save Eyvel, and later discovers that the only way to do it is to use the Kia Staff, which can only be wielded by Archbishop Manfroy's descendants.

At Safy's request, Leif arrives in Tahra and provides aid to Linoan, but they are later forced to flee when the Thracian army captures the city. Afterwards, he leads his army to Leonster and liberates it from the Grannvale Empire's soldiers. At the end of Chapter 18, he decides to send half of his army to Alster to aid its civilians in their rebellion, which results in failure and the death of one of his advisers, Dryas. With his weakened forces, Leif spends the next six months fighting to prevent the imperial army from retaking Leonster Castle. He eventually receives aid from Seliph, who continues his effort to defeat King Bloom and liberate the kingdoms of Northern Thracia. Meanwhile, Leif travels to Munster, where he obtains the Bragi Sword and uses it to defeat Raydrick. In the Final Chapter of Thracia 776, he defeats Veld and liberates Munster, thereafter joining up with Seliph's forces to fight against Thracia and the Grannvale Empire.

During Seliph's invasion of Thracia, Leif learns from Finn that Altena is still alive and has been raised by Travant as his own daughter. In Chapter 9 of Genealogy of the Holy War, he informs Altena about her heritage as the princess of Leonster, causing her to demand the truth from Travant and later join up with Seliph's army.

After the Liberation War, Leif becomes the king of the New Kingdom of Thracia, consisting of the entire peninsula. He canonically marries Nanna, as revealed in Thracia 776, although he can fall in love with most of the other female characters instead in Genealogy of the Holy War. In Thracia 776, it is mentioned that his name will never be forgotten as long as history exists.