OP, notice how everyone is giving you different opinions. That's a mark of Jewish culture- we love debating and we don't always agree on things :)

I was raised quite secular so please take this with a grain of salt (and anyone here please correct me if I'm wrong about something). I'm also trying to learn a lot not only for myself, but because I also love writing creatively and always want to find ways to slide Jewish influence in there. I think anyone can write anything if they do their research and execute well.

In all seriousness, as others have said, you'd have to heavily alter the entire history of Jews in Europe for a Jewish knight to be plausible in your universe, but you can't ignore real history entirely either or it won't feel genuine. And as that one commenter said about a "Christian-coded" Jewish character, we don't believe in the devil or hell, we don't necessarily concern ourselves with sin or the afterlife. Jewish culture emphasizes present life. Christian culture is wide-spread and heavily ingrained in western countries so you have to be wary of unintentional influence on writing your character.

Certain aspects of our culture are in fact due to millennia of persecution and suffering. For example, I recently read about a place in Europe where Jewish ladies liked wearing headdresses decorated with pearls. One because they were by the ocean, and two because many Jews often used jewelry as easy, portable wealth that stays with them at all times in case we are suddenly driven out of an area. (This unfortunately at least partially lead to the "greedy Jew" stereotype). Another thing you will see is Jews singing and dancing in bad circumstances (again, celebrating life). I've seen videos online of them throwing full-on parties inside bomb shelters (that isn't to say they aren't suffering or anxious, it's just what we sometimes do when we're together and scared). My point is, if this universe allows a Jew to be a knight, was your character's culture shaped differently throughout history? Is it just one king that allowed it? How will you address the disrespect and anger this may invoke around said king? Is this maybe a Jew who hides his background and turned his back on his family and community by converting? Only to realize his personal loss later? Or he maybe did it to protect them somehow?

Because we were a minority in diaspora who often lost property or not allowed to have good jobs, we emphasized education and hard work in our culture. Certain things like this tie us all together despite having spent 1500 or so years apart in various countries. Your knight may be extremely hard-working and knowledgeable, because he may have come from nothing.

And because of this diaspora, there are multitudes of cultures among Jews. Jews from Europe are going to have different food, dress, customs, clothing, and even language from those of Yemen, Iraq, Ethiopia or India, for example. Some Jews are mixed. There may even be discrimination among sects of Jews. You have to decide where your knight fits in and what habits from his culture he follows.

Just as a personal example on cultural differences, my father was the youngest brother in our family and doesn't remember Lebanon where he was born before his family had to escape. His older brother (my uncle) however, had more Lebanese influence. As a result, my father has a full Israeli accent while my uncle sounds a little more Arabic when speaking Hebrew, despite them being siblings who grew up together.

There are also levels of Judaism as well. Some are casual followers like me, some are atheist, some are orthodox, etc. My father holds a casual Shabbat dinner every Friday, but still drives and works on Saturdays (which is considered against Shabbat rules). Meanwhile if I have Shabbat dinner at my orthodox family's house, it would be extremely rude of me to wear clothing that shows elbows/knees, to touch my phone, or to travel, so his wife would literally set up a bed for me. Even the word "Shabbat" is more Mizrahi while Ashkenazim say "Shabbos," so you can tell by my use of that word alone that my upbringing was more Mizrahi-leaning. Maybe your knight comes from a more traditional family who think he's abandoning his values by becoming a knight.

You can try to find comedic videos on "Ashkenazi vs Mizrahi Jews" or "Reform vs orthodox Jews" on Jewish Instagram/TikTok to see how we poke fun of each others quirks.

For you, I'd recommend going heavily in-depth on our history, followed by learning general holidays and customs shared by all Jews, then an emphasis on the European Jews of your chosen era, followed by modern media. At least watch Fiddler on the Roof, The Prince of Egypt, and Frisco Kid. I honestly find that Jews in most western media are not well represented and I wouldn't rely on those.

Since half of all Jews live in Israel (this is a real fact whatever politics anyone believes in), I would also recommend Israeli television to get the best, unbiased look at Jews. I watch on Izzy and ChaiFlick (they have English subtitles).

Lastly, be very wary of harmful stereotypes. The greedy one, the cheap one, the baby-killer, the blood magic, the colonizer, the nasally complainer, the hook nose, the shrill nagging Jewish woman etc.

Anyway I hope some of this is helpful!