A 16 year-old orphan and first year high school student who temporarily fills in as an assistant for SPR to repay her debt when she accidentally injures Naru's regular assistant, Lin, and breaks a camera that was being used in an investigation of an old building at Mai's school (It is later revealed that the camera was covered by insurance, and the reason for her employment is unknown). After the conclusion of the first case, she is offered a regular part-time position for SPR and acts as an office and on-case assistant for SPR after developing an interest in the handsome boss of SPR, Shibuya Kazuya who she nicknames Naru for his narcissism. She openly dislikes Naru for his egotistical attitude and often argues with him, but harbors romantic feelings for him as well. She believes that her attraction to him manifests in her dreams as he often appears in them smiling and supporting her. It should be noted that both her parents have died: her father when she was little, and her mother when she was in middle school. Mai was taken in by a teacher who helped her through those years, yet when she reveals this late in the series, she is living alone and supporting herself through her wages at SPR.
Naru determines that she has ESP abilities through a test (although she does the complete opposite of what she is told to do: instead of predicting which light would light up, she predicts which wouldn't, and has a miss rate of 1000/1000). Naru refers to her abilities as "latent sensitivity", which include her dreams acting as a form of precognition and her seeming clairvoyance, akin to an animal's ability to instinctively detect danger. In recent episodes, Mai has also experienced "out of body" movement, when she gives the key to her old home to Masako while she is dreaming. Whenever she is having these dreams, it seems that Naru was always there to guide her.
In the novel, it was confirmed that it was actually Naru's dead twin brother who shows up in her dreams, smiling and always guiding her. However, unlike in the novel it has not been revealed that the man in Mai's dreams, the person whom she believes to be Naru is not Naru, but his twin brother. In the ending of the Evil Spirits series (wherein he found his dead twin brother's dead body), Naru is about to leave for England when she confesses her feelings for him. When Naru asks if the one she likes is him or Gene, Mai cries and is surprised to see Naru is still there when she stops. He tells her that whether she happy about it or not, she will eventually see his brother again (after they found Gene's body, she stops dreaming about him). Two months after Gene's funeral, Naru and Lin return to Japan since there are more supernatural activities going on in Japan. At this, Mai becomes his assistant once again.