Since Lipsha Morrissey narrates chapter thirteen of Louise Erdrich's Love Medicine and undergoes a kind of moral and physical change, he can properly be called the protagonist of this chapter.
This chapter is called "Love Medicine," as is the novel, and Lipsha is the one who claims to have the gift of healing in his hands. It is true that using love medicine (the rather tribal belief that using tokens and other talismen will cause people to love one another, even against their will) is his grandmother's idea, but it is Lipsha who practices it. Near the end of the chapter, Lipsha says:
I had gotten humble in the past week, not just losing the touch but getting jolted into the understanding that would prey on me from here out.
Because of this sense of moral enlightenment, Lipsha is the best candidate for protagonist in this chapter.
The more difficult assessment is who the antogonist might be. The only other significant characters in this chapter are Marie Kashpaw, Lipsha's adoptive grandmother; Nector Kashpaw, her husband; and Lulu Nanapush Lamartine, the woman with whom Nector has had a long-term affair. One could make a case for all three of them, in some way.
Marie is the inadvertent but clear cause of her husband's death, She is the one who insisted on using love medicine to make her husband be more faithful to her. Without her, Nector would not have had a raw turkey heart in his mouth; and if she had not thumped him on the back to make him swallow it, Nector would not have choked to death. She could be the antagonist, though an unwitting one.
Nector could be the antagonist, as well, since it is his cheating ways which prompt Marie's jealousy, which in turn prompts Marie's desire for love medicine. He is stubborn and does as he pleases (thus his unfaithfulness), and he is the only to blame for his own choking since he wanted to taunt his wife by taking in the turkey heart but not eating it. This is a direct insult to her love and his commitment to his wife.
Lulu has a minor role in this chapter, but she plays a major role in the failure of the Kashpaws' relationship. She knowingly conducts a five-year affair with Nector--in the same building in which the Kashpaws live. She is not subtle nor is she kind. Perhaps she is the antagonist of the chapter.
The most likely candidate for antagonist is Nector Kashpaw. He creates his own problems and is therefore responsible for his own death.