Here is a quick review of what a co-signer is, why you might want to apply with one and what qualities a co-signer should have to be effective.
A co-signer is not an occupant. This person is added to the lease on behalf of one applicant or an entire household as the sole guarantor of the lease to provide added assurance someone will be responsible for rental payments should a household, or member of the household, be unable.
This is a great way to strengthen an application should credit, income or eviction histories be an applicant's concern. To be most effective, a co-signer's credit score should be in good standing and above that of the owner's credit expectation for applicants. A co-signer's proof of income should reflect sufficient funds to cover both their own expenses and the applicants'.
Whether your co-signer will be responsible for just your portion of rent or the entire household will be a decision made among your household and your co-signer(s). This will be confirmed and acknowledged upon your lease signing.