The protagonist of the light novel Fate/Apocrypha. He was originally a homunculus made by Gordes to supply mana, created not to think anything and fated to die after having his mana wrung out by the Servants, but… Almost all details about him were depicted in the novels through the entire course of his life. After all, he has virtually no past to speak of.
Not only that, his life was set to be limited from the beginning as the premise of his birth. Even if he had thought of living a respectable life, he would only have live three years at the most. It’s precisely because of this that he continued to choose absurd, unreasonable and reckless choices for the sake of “what he thought he should do.”
Even after he acquired greater longevity, that aspect of him didn’t change. The reason Sieg acted for the sake of humanity at the end naturally wasn’t for humanity’s sake in and of itself. In the first place, Sieg hadn’t experienced enough to completely believe in humanity.
However, Jeanne d’Arc waged her life to win the future. It is because he saw that and wished to “become her strength” that he acted, a greatly personal desire.
Additionally, his Magic Circuits alone were of such high quality that Avicebron had an eye on him and were in no way inferior to those of Darnic, Yggdmillennia’s head. However, he can only use them to destroy things through Structural Analysis. Even if he had survived, he wouldn’t have had any intention of studying magecraft since he doesn’t have a very positive image of it.
Having transformed into one of the Dragon Kind at the end, Sieg has already lost his physical body. His final state is the form of Sieg’s soul after he transformed by making a wish upon the Holy Grail.
Incidentally, the words Jeanne spoke at the end of the novel made Sieg finally understand the reason why he continued waiting for her all that time. The two of them take being thick-headed to a new extreme!

Fate/Apocrypha material: Fate/Apocrypha Encylopedia