Who is the antagonist that challenges Denny in custody battles?

The character that presents the most significant challenge to Denny during the custody battles is Denny's father-in-law. This relationship is fraught with tension, particularly because Denny’s father-in-law is determined to gain custody of Denny's daughter, Zoë, after the death of his daughter, Eve. His motivations are rooted in a desire to protect his granddaughter and perhaps a lack of trust in Denny’s abilities as a single father, which creates a powerful antagonist dynamic in the story. The conflict over custody reflects deeper themes of family, loss, and the struggle for what is right, making Denny's father-in-law a pivotal figure in the narrative.

The other characters either do not have the same level of antagonism toward Denny in this context or do not actively challenge him in the custody matter. Kevin, for example, is more of a supportive character, while Enzo, the dog and narrator of the story, provides emotional support rather than conflict. Denny's own parents, while they might have differing opinions, do not actively engage in the custody battle like Denny's father-in-law does. This makes the father-in-law the most appropriate choice as the central antagonist in this situation.