Board Thread:General Discussion/@comment-24100805-20160201152915/@comment-32182236-20180315231834

Frisk IS gender-fluid, because they're whatever gender you want them to be. Just like they're just as nice (or murderous) as you want them to be. You, the player, roleplay Frisk.

As a result, both male, female, and non-binary are all correct interpretations of Frisk. Just like Pacifist Frisk and Genocide Frisk are both real interpretations of Frisk.

Of course, since "they" is normally used when gender is unknown, the game uses it, and the Wiki also uses it for articles, as to not take sides, which would break the neutral point of view.