One important thing is their writing style. It should match the tone you want for your story. For example, if your story is light - hearted, you don't want a writer who is overly serious and formal. Also, check their experience. Have they written similar stories before? And of course, look at their portfolio to see the quality of their previous work.
When choosing a writer, you should consider their availability. You don't want someone who is too busy with other projects and can't give your story the attention it needs. Their communication skills are also crucial. They need to be able to listen to your ideas and collaborate with you effectively. Another aspect is their creativity. A good writer can bring new perspectives and ideas to your story. Look at how they handle different themes and characters in their past works.