After escaping and receiving the hero Siegfried's heart, he becomes notably more selfless and brave as he gradually continues to develop as a person. Of his own volition, he decides to forsake a life of happiness, and instead aims to fulfill the desperate wishes of freedom his brother and sister homunculi who would die if nothing was done to save them. Eventually, he even willingly becomes a participant of the Great Holy Grail War in return for their promised freedom and safety as well as for the sake of both Astolfo (who had previously saved him) and Jeanne (who protected him). Due to his young age, he is initially very much naive and clueless about things such as common sense, basic human behavior, and how the world works though he is able to pass as an eccentric but otherwise normal, if slightly clumsy, human with a calm and logical personality in a very short amount of time. While typically very polite and humble, Sieg can at times display a blunt attitude during the rare instances he feels annoyed or frustrated. He also has a tendency to blurt out slightly rude observations without really meaning to which occasionally gets him into trouble with others.  He seems to have a high capacity for forgiveness as shown when goes as far as to unhesitatingly throw himself in harm's way to save both Caules and Fiore when their lives are in peril despite the Yggdimillenia's treatment of his brethren