If you are not asexual and you want to write an asexual character, start here: You need to actually write an asexual character.
Don’t write an asexual character as yourself minus something. Don’t write an asexual character as a person with a missing piece. Don’t write an asexual character as if their orientation is a hole with nothing filling it. Don’t write an asexual character as if they’re you after you lost something, or you with something you treasure scraped out of you.
Write the asexual character as a person who grew whole without that part, but don’t think of it as a part of them that should be there or used to be there or will be there someday to complete them. Writing asexual characters this way is what makes so many of them feel robotic or alien–it’s the same thing that makes ignorant people ask asexual people in real life if we’re fully alive. Don’t write around a hole, and don’t fill it with a substitute. Write them fully realized, as a person. Yes, depending on the character they may have been hurt, or they may have very serious problems, or they may have a tough time finding happiness (even partially because of their asexuality, like many of us do in real life). But please, write us as people.
As people. Not as ideas with missing hearts.
How?
Talk to asexual people. Read work by asexual people. Ask asexual people to read your work and evaluate your characters.
But most of all? Don’t think of yourself as the baseline and them as defined by how they aren’t like you. Write about them. What do they have?
Know what they have, and write about that.