Hi! Hope you´re doing great. I come from the poll but I wanted to comment here anyway. I think being gender-neutral is the best option, it can suit anyone and is less work for you lol. If someone wants it more customizable I´d give a choice at the beginning of the game (The typical "What are your pronouns?" "she/her" "he/him" "they/them") But leave it as a dark shadow/person anyway ´cause again is less work for you and it´s hard to satisfy everyone so it´s better to keep it simple.
And let´s be real there aren´t many Undertale dating sims as good as this one looks, I really doubt someone is going to be picky about the MC so don´t worry much about it! At least I´m not gonna be picky at all. I´m too focused on the characters to pay attention to mine lmao
Hope it helps! Don´t doubt to ask for advice if you feel like you need it tho xoxo