It depends. Consultants provide advice or services, e.g., supplying software, providing technical assistance or training, or setting up equipment. They are listed as Senior/Key Personnel only if they will contribute substantively and measurably to the scientific development or execution of a project.
Otherwise, consultants are treated as Other Significant Contributors—and as such you must Create Biosketches for them in your application, even if they will not be paid a salary from the grant award.