FirstName
LastName
https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/sforce_api_objects_user.htm
2 answers
Standard fields can be a bit tricky. The ID of the owner is OwnerID, but to use it as a relation, you need to use Owner.FirstName, or any other field belonging to the User object (https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_user.htm).