Board Thread:General Discussion/@comment-24100805-20160201152915/@comment-32182236-20181127150042

No matter which gender you give Frisk, something like this is going to happen. If they're a girl, you get a girl wearing a cowboy hat. If they're a boy, you get a boy wearing a tutu.

So, it's pretty clear they're not 100% canonicaly a boy or 100% canonically a girl.