you flirt with whoever for the first few chapters to get a better feel of their personalities, so yes you can flirt with them all technically. Then in/after chapter 4 you get the option to pick a romance route for only one character. You also get the option to enter into a poly relationship with more than one character (I think) like if you choose Alkar's route, since him and Omen are close you can enter into a relationship with both of them.