What do you think are fundamental differences between Aleksander and Kaz? On an even playing field i.e. comparing a young Aleksander to a young Kaz and the very old Aleksander to your interpretation of what a very old Kaz would be like.

I haven't read the SoC duology but from the few things I know about Kaz, the most important difference between them is the matter of selfishness. Aleksander is selfless in comparison to Kaz and has a more important goal than gaining money. He tried to save a group of people and a country from various enemies for hundreds of years while Kaz always is on the look out for the next job that will fill his wallet.

Both of them had a childhood that shaped them to the men that they eventually became. Kaz lost the only family he got while Aleksander was living on the run with his mother (and Baghra was not a pleasant, immortal companion to have). The environment they grew up in formed their future plans but in different ways:

- Kaz grew up among thieves, street magicians and crookers and ever since his brother died he had nothing left to lose. His feud with Pekka Rollins gave him a lifelong grudge that became an essential part of his character.

- Aleksander grew up with his mother travelling from place to place, never safe because of their powers. Just like Kaz, his incident with Annika shaped him to the man we came to know in the trilogy but unlike the former, he decided to act on a larger scale because he was a witness to the Grisha persecution.

So even though both were scarred because of their childhood they decided to take different actions in the future.

Now, as we've seen with Aleksander in his later years, he was still fixed on his goal but he had grown bitter with life and more apathetic in comparison to his younger self. He was a pragmatic and action-oriented person that craved companionship and when he had his chance to have that he behaved with panic at first and then with gestures that showed that he no longer had an idea how to go for it in a normal way.

With Kaz we see the opposite. In the beginning he's distant and cold with his friends, behaving in a business-like manner but as his own story progresses he tries to become more open, especially with Inej. He tries to find his footing in being more empathetic for her sake and I believe in the future he will succeed. He won't be as closed off as he was the first time he was introduced to us. But he has a long way until then (it's the trauma he has to get past you know).