As I've also mentioned, they are both New Yorkers ... well, the vampire has lived in my fair city for about 25 years, so that confers some sort of status of citizenship.
The human was about five or six, and got to watch it live, and in person, in the borough across the river, Brooklyn. Yup. My character watched the whole thing happen live. Even remembers the snow on the ground that wasn't even snow. It was ash.
The vampire remembers 9-11 just as well, but couldn't go outside until the towers fell. The ash and dust blocked out the sun, and, well, a vampire didn't really need to breathe in all of that mess, so could go outside at that point.
But before that, the vampire could see the towers fall over and over again on tv, because cable made it possible to watch the second plane strike over, and over and over again, because this was before everyone having an iPhone, and we didn't see the first hit until later.
The vampire understood what was happening perfectly well, and wondered how many thousands dead wouldn't be reported, because vampires went up in flame and fire and sunlight from broken windows.
One would grow up to become a monster. The other wished that vampires were bigger monsters -- and hungry for Islamofascists.
Before 9-11, I had figured that Islamofascism would die a slow death, along the line of Darwin, Yes, as I stated before, I would have been happy to have offed Saddam in the 90s, or invade Sudan to stop the ongoing genocide, but I figured no one would do it for purely humanitarian reasons.
After 9-11, kill 'em all.
Will it follow me and all of my characters for all the days of my life? Maybe. I honestly don't know. It's possible. Codename: Winterborn would not have happened without it. And I can honestly say that Honor at Stake wouldn't have happened without it. Not only has it affected me, and the characters, but it shaped the story the first time out.
I have a space opera that I wrote back in the 90s that has undergone some serious revision as far as the bad guys are concerned. It's not entirely influenced by the evil of 9-11, but there are echoes and elements here and there.
Victor Davis Hansen, in his book Ripples of Battle, examined how small battles -- some that people never even heard of -- have lasting effects, touching all sorts of things. With 9-11, it might well have created the next generation of politicians out of vets from the sandbox. Maybe you have a few people who lost someone, and they become a President of the United States who nukes Mecca. No idea.
But I know that 9-11 has definitely shaped these books, and my characters, and large parts of the way I think and approach things.
And, right now, I also know that I will never forget. I don't think my brain will let me.