s it okay to make up your own surnames?

Yes it's perfectly fine cuz my friend did it 2 :) good luck

Sure, it's your story and your imagination. If it makes sense with the character and blends in with the story, または just..something inside あなた tells あなた to use it, then go right on ahead :)


There's a saying I always have when it comes to writing;
"Trust the gut"
If there's any idea that suddenly strikes あなた and あなた from within that it's a good one, trust yourself. And go for it :)
Think about it if あなた wish to, but I'd suggest that あなた listen to yourself when あなた have some type if intuition like this. ^^