A cosigner for an apartment takes on the financial responsibility for a lease. Learn how having a cosigner can help someone get approved for an apartment.A cosigner guarantees that if the tenant fails to pay the rent, the cosigner will step in to meet the obligation. Rather than writing a new lease entirely, you can simply add an addendum. Having a cosigner for a lease can help you rent the apartment of your dreams but also divides your rights and responsibilities as a renter. A cosigner is someone who signs the lease along with the main tenant of the rental property. Com or ask your property manager about Leap. All co-signers must be based in the United States. Print out the Co-Signer form (2 pages), and fill it out completely. This includes the responsibility to pay rent and follow the lease agreement closely.