For those who are familiar with the LA rental market, do landlords typically require that the co-signer be physically present when signing an apartment lease? What is a cosigner in Los Angeles?Com or ask your property manager about Leap. Learn the essential steps on how to add a cosigner to a lease to strengthen your rental agreement. To cosign, the person must have a clean rental history and not have gone through any evictions. A guarantor or cosigner for an apartment could help you fill financial gaps when applying for your rental. Find which is right for you. Cosigners sign the apartment lease with you, taking on the same financial responsibility as a tenant and then some. Cosigners and guarantors sign the lease with you and fulfill the purpose of taking responsibility for the monthly rent. So, in the event you do not qualify on your own, your landlord may ask you to get a cosigner or a guarantor to complete the rental application.