Now, there are some things that canon doesn't expand upon for characters because they're minor, trivial... Or maybe the character just gets a couple minutes of screentime, so there's really no point for the creator.
These sort of things could be stuff like favorite foods, books they've read... In this case, how good of a flirt they are. In the end, that sort of thing is a person's personal headcanon. For example, in my headcanon, I imagine Zexion as being quite the capable flirt when he has reason to. (9 ties out of 10, those reasons will be manipulative in nature.) I think this because his skills with illusions and general trickery could hint at a very good acting face.
However, I'm pretty sure even in my headcanon, Zexion wouldn't be able to make a random stranger fall in love with him just by a glance when he's among hundreds of other young men in a crowded concert hall that some chick's Sue is singing to. I'm also doubting that in the almost literal minute they have of mindless chatter backstage (seriously, it was 'want to come to my house?' 'okay!' and that's it) that he could further this.
The Sue's writer tried to convince me that the whole thing where their eyes met during the concert was "flirting", but... Well, you know: I don't think that word means what she thinks it means. Although really, that was the least confusing thing in the
fic.
I like to imagine that hypnotism was involved and that Zexion is really just trying to get close to her to steal all her money and various other expensive things.