Honestly, If you see them as a girl, that is fine, don't worry about it, it's not really a big deal, i see Kris as a boy, Frisk as a boy and Chara as a girl, so i draw them like that, it's just an opinion after all.
It just occurred to me u might be talking about Riley, not Kris XAX;;;; Oops! Sorry 8'''D got ahead of myself But, yeah! It was established in Chapter 1, though I understand with me starting uploads at Chapter 3 it probably skipped by a lot of people 8w8
Yeah 8w8 I figure kris/frisk(/chara) were always meant to be very gender ambiguous so that the player could self insert onto them, in the same way their actions (aka, your actions) define their personality instead of words. That's why its frustrating when people think they're male, which is always sort of considered a false 'default' >A<;;;