If a debt is co-signed, can I protect the co-signer?

You can protect co-signers in Chapter 13 if you provide in your plan that you will pay the debt in full with interest through your case. (Alternatively, your plan could provide that the co-signer is going to pay the debt and that you will not pay it in your case. This may not be advisable in your specific case, and would not protect the co-signer if the loan were in default).
In Chapter 7, co-signers are not protected. (This is one reason why Chapter 13 is sometimes to be preferred over Chapter 7). Thus, if you and a co-signer both owe a debt, and if you file Chapter 7 in your name alone, then you (as “the debtor”) will be discharged, but the non-filing co-obligor will continue to owe the debt. Of course, if you are keeping (“reaffirming”) a debt in Chapter 7, then the co-signer will be “protected” under state law so long as payments are not in default.