Zagan

An 18-year-old mage (and, later, Archdemon) who inhabits the territory of Kianoides. In his youth, he was kidnapped by the mage Andras to be a Human Sacrifice. At that moment his latent magical ability awakened and he killed the mage, taking his place.

Zagan didn't seem to want immortality so much as he wanted a reason to seclude himself and calm down from his Traumatic Superpower Awakening. (He picked immortality specifically because it's what every other mage is studying.) It's not until he frees another slave that he realizes he can use his power to do things that really matter to him.Subverted later on as he starts to care for others. Constantly working and developing new spells in order to gain an advantage on his enemies and to protect his loved ones.