Meta Mato

ALT Kris shares a room with his brother Asriel. Generally speaking, it's often seen as socially uncommon if not completely unacceptable for teenager siblings of opposite sexes to share a bedroom, whereas siblings of the same sex sharing a bedroom is significantly more common. While it's not impossible for teens of different sexes to share a room, the fact that no one acknowledges this abnormality may hint that Toby Fox hadn't put much thought in regards to that detail.

ALT Kris is shown to have self-esteem issues in regards to his stature and physical strength, as see when he attempts to use Queen's arcade cabinet. Height and strength are traits generally valued more in men, with taller and stronger men often being seen as "more masculine". If Kris was "non-binary", it wouldn't make much sense to include a scene where he displays what some might call "fragile masculinity" as being masculine wouldn't be a concern for him.

ALT Kris is well liked by Carol. From what we've seen of her, Carol Holiday is very a strict and conservative parent, as well as possibly homophobic. It's made very clear that she does not approve of Noelle and Susie's relationship, and that she instead views Kris as Noelle's ideal suitor. If Kris was "non-binary", realistically Carol would be just as, if not more critical of Kris' relationship with Noelle, but instead she welcomes him with open arms. You could argue that this is due to Susie's reputation as a "bad girl" rather than Carol not approving of Noelle's lesbianism, however Kris has repeatedly been described as being a troublemaker growing up, so this is unlikely.

ALT Aqua is shown to be not fully grasp Seth's more clearcut gender identity, asking if they're a boy or a girl and then calling them out for being confused after Seth dodges the question. Aqua does not however ask this question to Kris despite him supposedly being a similar case to Seth, instead being more interested in his blue skin and "human" abilities. Interesting, there does exist cut dialogue that would have drawn a parallel between Seth and Kris, in which Seth asks Kris to back them up after comparing themselves to him, however as this dialogue is cut, it might as well not exist at all.