Rosie Jarman, age 30, is a doctoral student of psychology and a bartender. Her character operates as a foil for Don’s because they are opposites in nearly every way. Where Don is rigid and prefers logic over emotion, Rosie likes to take things as they come and is regularly overcome by her emotions.

Rosie finds Don funny, interesting, and charming because she is an open-hearted, straight-talking, flexible person. Brilliant and somewhat eccentric herself, she is more accepting than others of Don’s seemingly rude behaviors. As she gets to know him, she develops an attraction to him. Feeling that his rigid ways hold him back, she tries to get him to loosen up. Being a spontaneous, fun person, Rosie enjoys taking Don out of his routine, and the trip to New York offers her a unique opportunity to take charge and push him out of his comfort zone. 

But while Rosie values Don’s extreme kindness in helping her with the Father Project, along with his improved ability to be a good companion, she is frustrated by his struggle to understand and react to social and emotional cues. In New York, she works to temper her reactions when Don’s obliviousness causes friction, but she reverts to her old habits when Don assumes she has no interest in him. Rather than consider how he’s processed everything that’s gone on between them, she perceives it as a rejection and angrily tells Don that he could only be a perfect partner if he changed himself entirely. This inability to trust Don stems from a near-lifelong emotional trauma of not knowing who her biological father is. Feeling abandoned by this mystery man and struggling to connect with her supposed stepfather Phil after her mother died, Rosie tends to have complicated feelings about relationships overall. At the first sign of trouble or perceived dishonesty, Rosie lashes out rather than taking the time to assess the situation. In this regard, Rosie finds emotional expression as challenging as Don. Both of them struggle to emote properly, but whereas Don’s issue comes from suppressing his emotions, Rosie can’t seem to get hers under control when it counts.

Rosie’s character develops over the course of the novel. Her major change occurs when she sees that her rejection of Don is cruel and based on unrealistic expectations. Once she realizes that she loves Don as he is and only wants him to change if he feels it’s right for him, she accepts his proposal.

Head of the psychology department, Gene, age 56, is an aging womanizer who uses his research into human sexuality as an excuse for his philandering ways. His wife, Claudia, has agreed to be in an open marriage. However, this carte blanche to sleep with whomever he wants eventually drives a wedge between Gene and his wife, as well as between himself and the world. He is not respected for this behavior, as he is too blatant in his lack of commitment. In his own way, he struggles as much as Don in the realm of relationships. 

Gene is also Don’s best friend. For most of the novel, Gene guides, advises, and teaches Don about how to socialize and navigate the world. Though Don admires Gene’s success with women at the beginning of the novel, as he gains confidence and insight, he comes to see Gene differently. Their relationship is saved and even evolves when Don points out how unacceptable Gene’s behavior is. Don’s confrontation marks a turning point for Gene. His response to Don’s critique of his behavior shows that he values Don’s opinion, and it affirms his commitment to his wife because he reevaluates his behavior to preserve his marriage. Without Don’s interference, Gene may not have changed course in time to save his relationship with Claudia.