A guarantor is a person or a company who will co-sign on your lease in case you do not qualify to rent an apartment in New York on your own. In simplest terms, a guarantor is a tenant who does not live in the apartment.My DD just graduated and is going to rent an apartment in NYC with 2 other roommates. The lease starts after she will have only one pay cycle at her job. Postgraduates often turn to a parent to serve as a guarantor.