See, I don’t really care that the player is referred to as “They/Them” in Deltarune, because it’s established that you are playing as Kris and those are their pronouns.

(and in Undertale, the monsters simply had no idea what Frisk’s gender was due to an unfamiliarity with humans so it’s kept intentionally vague… with they/them simply being the most gender neutral thing to call them and the fanbase having their own headcanons on what Frisk actually is. Personally my head canon is Frisk is male and Chara is female, which seemed to be the most common interpretation in the fanbase back then)

Sidenote: First non-binary person I ever met used ey/eir pronouns, this was so long ago that ey called eirself “Genderless” instead of Non-Binary as the latter wasn’t a word. Ey was femme presenting, but very much not female. Sadly we’ve drifted apart and wherever ey is I wish eir well.

I always wished ey/eir had caught on instead of “singular they”, because personally I thought “ey” sounded cooler and was more straightforward than “singular they”. But hey I’m not non-binary myself so it’s not really any of my business.