We use singular 'they' in two ways:
- when we want to refer to a person whose gender is unknown or irrelevant;
- when we want to refer to a person who doesn't (want to) identify as either male or female.
When you use 'they' as the subject, whether you use it as a 'singular they' or 'plural they', ALWAYS use a plural verb.
Kathrin is a non-binary person.
Though we are talking about one person, but we are using the singular 'they'. Therefore, we use the plural verb 'are' not 'is'.