The protagonist in my story is aro, i need some writing tips

[Guest Melora Sumi]

My main character is female and is seventeen, she's also aro and a hardworking, intelligent, noble magician. Being aro isn't the main focus since the story is fantasy. She is surrounded by her friends who love and support her. Her friends know she's aro and respects her. There's a part of the story where MC saves three princesses and returns them to their mother. Their mother is very grateful and offers her any of her daughters' hand in marriage. What do you think is the best way for MC to tell the Queen she rather be friends with the princesses instead without sounding rude or ungrateful?

[ScarfOfSexualPreference]

Everything tempesta said is super right, but I’d also add that you should think about the character’s personality. Is she ashamed about her aro-ness, or has she accepted it and is out? Because of her personality, she may try different ways to turn them down. For instance: describing the truthful situation, saying she’s not interested in a relationship, saying she’s already in one, lying about her sexuality or romantic orientation to save feelings, or just politely declining without a reason. 
hope this helped! 

[roboticanary]

The answer to that depends on the character and her personality. As much as it is important she reacts in a way which does not sound ungrateful the most important thing is that she acts as a person. She should have a real personality, real feelings and act in ways which are consistant.

If she is a shy character maybe she offers to talk it through with the mother in private, rather than an open refusal. when they are alone she explains that she has no interest in marriage. If she is a passionate character maybe she does state outfront that she has no interest in marriage to any of the princesses. Perhaps she uses a bit of flattery to make herself sound less rude. If she is known to be very compassionate to others perhaps she argues for the right of the princesses to choose their own path in life even if that society does not often consider it.

If she is trying to be polite perhaps she asks if she could instead have something that would be of use to a magician. That might tone down the problem of sounding ungrateful.

One last thing it is worth thinking about is why the mother would consider turning down the offer to be rude. If the mother is annoyed because she thinks the refusal is an insult to her daughters beauty perhaps your adventurer could respond to that. Is the mother annoyed because she has turned down an offer that would give her great power, maybe that makes her think of refusing as a slight to the family. If that is the case then your character might try a very different way of not sounding rude.