Heyaa, it's me again lol just wanted to give my opinion on this of it's Okey :)
Gender is a pretty complex thing so my opinion may (and probably will) differ from others xD but personally i would tell you to choose based on what masculinity means to Rune. By that I mean not only the way they sees themself but also how he relates and interacts with the world around their own view of masculinity and gender. Personally i do think there isn't much representation of demi boy in ifs, generally people go for they them (nothing wrong with that obviously) but it can also be a bit complex trying to represent a non-binary person who still feels represented by certain biases of a specific binary gender
Also! I said demi-boy but it could also totally be another kind of polygender as genderfluid, so this should be another thing to have in mind when building their idea of gender.
Another idea you could go by could be they (or he) doesn't really know, I mean, how old are they? I doubt they are older than our bro and he is 16 iirc. With 15 yo or less no one really knows themselves well enough and in a world without the fastest exchange of information like ours thanks to the internet, I don't know if they have a lot of sources about gender. You can start playing with that to start a lil character development of them even that could be part of the relationship with them (romantic or platonic, like MC being someone from our world should know a bit more about gender and identity, so they could help a bit).
If you want my personal opinion, I would like to continue seeing them as a he/them, I don't really saw a lot of ifs with this kind of representation but if you choose other thing it's totally fine (at least for with me xD) too
Sorry for all this mumbling hahaha I hope I help you even a bit!
Ps sorry if there is some mistake or typo in there, English is not my first language so sometimes it is a bit difficult to express myself clearly enough haha
Hey! And of course your opinion is welcome - I did ask for it. (I'm also always open to hearing other point of views, especially on topics which aren't exactly easy. There are a lot of experiences that I simply don't have and a lot where my experience isn't universal so hearing other angles is always helpful.)
You've made some excellent points and gave me some good questions to consider. It definitely helps. :)
I slept on those questions and did some trawling through reddits and blogs of people talking about their own experiences with gender.
Honestly, Rune's relationship with masculinity is rather similar to the one I have with feminity. Which tells me that the right answer for him is "he/they" - because the right answer for me and my 'sure, that's mostly right but it's at an angle and a tad wonky and a step to the side' is "she/they". They're a little closer to "they" on the gender spectrum but not so close that tossing "he" out feels right to their character.
So, I think I'll go back to Rune using both pronouns after all. And go check how other authors initialized their players using multiple pronoun sets too. Difficulty is no reason to not program this stuff in, and better to add it now so it's built in than have to try tackle it later.