Don Tillman
Don is the main character in the novel and also its narrator, so the reader is able to see everything that happens through the eyes of someone with Asperger’s Syndrome, which is the condition that seems to be the most rampant and dominant within Don's personality. He is also intelligent and self-deprecating enough to see this in himself and to recognize that it is something that makes him so different that he has trouble fitting into normal, average society and situations. Don is an expert in genetics and, until meeting Rosie, has viewed the world with an entirely scientific viewpoint. He is a lecturer at the university; although he is seen to comply with the letter of the law and has not acted in a way that would enable the Dean to fire him, he skates rather close to the line.
Don is approaching finding a wife with the same scientific methodology that he uses to approach everything else, and he is baffled by his lack of success. He has never experienced love before and is therefore wholly resistant to the concept that finding a wife is more due to unexplained chemistry and emotion than it is to locating a mate who is statistically likely to be compatible. He is completely logical and oblivious to what is socially expected of him; for example, he is so good at mixing cocktails that he notes that if he is no longer able to work at the university, a career in cocktail bar ownership is the only logical career choice, not comprehending the social implications of such a change. Don's list-making and minute-by-minute timetabling of his day, as well as his Standardized Meal Planning, make him a difficult and somewhat inflexible companion, but his willingness to change his entrenched patterns of behavior for Rosie is very endearing. Because of his total logicality and his inability to understand what is expected of him, he makes a terrible alibi and, on many occasions, lets it slip to Claudia that Gene has either not been with him or not been where he is supposed to have been. Don is an unlikely romantic hero, but the reader does nonetheless find themselves rooting for him and for the success of his "relationship" with Rosie.
Rosie Jarman
Rosie's general and deliberate appearance gives off an air of aggression that does not reflect the delicacy of her psyche: she is, by her own admission, messed up by the uncertainty about the identity of her birth father. For this reason, she has spent her entire life finding fault with everything that Phil, the man she believes to be her stepfather, has done for her, noticing the one time he did not come through on a promise (to go to Disney World) rather than noting all of the many sacrifices he made for her and the love he has shown her all her life. She is rather combative, but this hides her emotional neediness. She is surprised that she is attracted to Don but is also very aware that initially, before he decides to make some changes and become somewhat less intractable than he was when they met, they are incredibly incompatible: she is a smoker, a vegetarian, and a spontaneous woman with romantic tendencies; he is the opposite. Rosie often tries to downplay her attractiveness because she wants to be taken seriously as a person and not constantly be objectified as a woman and judged because of her job as a bartender. She is extremely intelligent, and although she has fought against the desire to follow her parents into the medical field, she nonetheless follows them to a degree as she is studying to become a psychologist. Rosie is a sweet and loving woman who wants to find a mate who loves her for herself and not for the answers she has given on a questionnaire.
Gene Barrow
Gene is a fellow professor and is also one of Don's best friends (the other being Gene's wife, Claudia). Although he is never diagnosed in the book, Gene appears to be a sex addict and has absolutely no regard for the feelings of his wife when he blatantly cheats on her. For an intelligent man, he is surprisingly unobservant and clueless when it comes to basic observations about the state of his own marriage. Gene's relationship advice for Don is usually useless as it does not contain much about emotional attachment or feelings, focusing instead on sex. His advice to Don when he takes Rosie to New York is to give him a book of sexual positions. Gene's goal is to sleep with a woman from each and every country in the world and he is pathetically proud of this as he displays a map in his office for all to see, with pins stuck into every country that has thus far been represented in his sex life.
Claudia Barrow
Claudia is Don's other best friend, and she is Gene's wife. Although Gene claims that she is happy to have an open relationship, this is really not the case, and she is fast coming to the conclusion that she does not want to continue in a marriage that is based on cheating. For a psychologist, she is very slow to recognize and diagnose her own feelings and emotions. She is a much better advisor for Don in his relationship with Rosie as she takes into account the mystery of emotions and also is able to advise him on the changes he might be wise to make if he wants the relationship to progress and flower.
Phil Jarman
Phil is a personal trainer. He is the man Rosie believes to be her stepfather, but he turns out to be her birth father. Phil is a put-upon man whose every effort with Rosie has gone largely unappreciated; he has, in fact, been a rather wonderful and giving father who has done his best to raise a difficult child with patience and love. He is forgiving and open to improving their relationship. He clearly loves Rosie very much, as witnessed by his obvious hurt when he hears that she refers to him as her stepfather.
Dean (Professor) Charlotte Lawrence
The Dean is not a big fan of Don because he never does anything that she can actually terminate his tenure over, much as she would like to. His refusal to understand and participate in the politicking necessary to obtain grants and support for the department is an enormous frustration to her. The Dean goes by the nickname "Charlie," but Don finds it impossible to equate this rather friendly and informal monicker with the fierce, unfriendly woman with whom he is frequently in combat. Professor Lawrence is dedicated to improving the University and the department. She is in a relationship with another female member of staff and has absolutely terrible fashion sense.
Bianca Rivera
Bianca is an accomplished woman who answers Don's Wife Questionnaire and, on paper, seems to be a perfect match for him. In reality, though, she is humorless, judgmental, and absolutely furious to find that Don is not the Fred Astaire on the dance floor that she requires in a partner. She is the proof, if proof were needed, the love does not happen on paper, but in real life—and in life, she is not a suitable partner for Don.
Daphne Speldewind
Daphne is the first real human relationship outside of his family that Don has been fully engaged in. Daphne is an elderly neighbor whom he befriends. She is ultimately moved into assisted living accommodations as she falls victim to Alzheimer's and is unable to remember him or their relationship. Daphne wants the best for Don and is constantly encouraging him to find a partner.
Simon Lefebvre
Simon is one of the candidates for Rosie's biological father, but he smells a rat when he receives Don's questionnaire and request for DNA. A friend of the Dean, his suspicion initially endangers both the Father Project and Don's tenure at the university, but fortunately, the fake project that Don has described in order to obtain Simon's DNA turns out to be a viable area for research and ultimately secures the grant for the department that the Dean has been dreaming of.
Kevin Yu
Kevin is an international student who studies at the university where Don works. Don catches Kevin plagiarizing an assignment, which puts Kevin in danger of being expelled. Kevin is working very hard to be able to emigrate to Australia and bring his wife as well; he is quite a good student but lacks confidence in his writing abilities, which is what leads to plagiarism. Kevin represents an example of Don's increasing emotional intelligence and ability to show compassion.
