What do we know about him except being a good friend of Benjamin Sisko, a diplomat and a Ladies' Man?
Pretty much that's plenty considering he's not really a character on the show. What do we know about others that aren't on the show?
He appears in some novels. Alas I can only see an old man with a symbiont given to Jadzia as we can see in one episode. I'd like to have seen him as young man, though.
He was the godfather of Kang's son, and he swore a blood oath with Kang, Koloth and Kor after the deaths of the latter threes' sons. Kor
He also lived longer than the other Dax hosts. Especially Jadzia wasn't blessed with a long live thanks to Dukat.
He was Sisko's mentor and they had some altercations during his young officer days. I know Curzon honored secrets, and he was quite manipulative to people when he wanted something. He tended to be mean spirited he was hiding something but can be as enjoyable to anyone who knew him or didn't. I think Curzon was one of the negotiators for the Khitomer accords; his efforts lead to the Klingon Empire having a relationship with the Federation. He's also great at playing Tongo.
Didn’t most of the Dax hosts except Torias, Jadzia and Joran have long lives? We know that Curzon, like Jadzia fetishized Klingon culture.
I think the writers always told us how awesome Curzon was and that everyone wanted to be just like him. All the men wanted to be him and all the women wanted him. Ugh. Klingons loved him, Ferengi loved him, who didn't love him?
Again, is there a question here beyond whatever can be looked up on Memory Alpha? (Remember that the year is 2019, not 1993.)
Everything can be looked up in 2019. Conversations are inefficient Curzon is confident, strong, intelligent, and charismatic, but also stubborn and selfish. He can be very intimidating and doesn't back down without a fight. Some aspects of his personality are revealed in greater depth in Facets (3x25) when he gets blended with Odo during a ritual where Jadzia gets to meet Dax's former hosts.