Can you pursue them all at the same time or does flirting with more than one upset each of them?
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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.