Warning: This section contains spoilers for all six books in The Mediator series!
Susannah "Suze" Simon was born in New York City and moved to Carmel, California with her mother when she was sixteen. Suze is a mediator, meaning that she has the ability to see, touch and speak to ghosts. She is the only child of Helen and Peter Simon. Her father died of a heart attack when Suze was six, but his ghost often pops in to check on his family. Her mother has since remarried, and she and Suze now live with her new husband, Andy Ackerman, and his three sons, Jake (whom Suze refers to as "Sleepy"), Brad ("Dopey") and David ("Doc").
When Suze arrives at her new house, she finds her room already occupied by the ghost of Jesse de Silva, a 170-year-old rancher who was murdered in her room in 1850. Although the two get off to a rocky start, they quickly become friends, with Jesse showing feelings for her almost immediately. The two form a close bond and help each other out of a number of dangerous situations, before finally admitting to each other that they have fallen in love. In the second half of the series, Suze's feelings for Jesse prove to be the major catalyst and driving force for most of the events that unfold.
After Jesse, Suze's closest confidant is the principal of her high school, Father Dominic. On her first day of school, Father Dom reveals to Suze that he is also a mediator. Although their mediation techniques differ greatly, Suze and Father Dominic usually work together when dealing with ghosts, and although he usually doesn't approve of her methods, he is always available to support and advise Suze. They quickly form a close friendship and almost familial bond.
Although she frequently mentions only having one close friend in NYC, a girl named Gina Augustin (who appears prominently in Reunion), upon moving to Carmel, Suze formed a number of close friendships. The most important of these is with CeeCee Webb, the school's newspaper editor and resident gossip. CeeCee and Gina are the only two people besides her stepbrother Doc who know of Suze's ability to talk to ghosts (not counting fellow mediators like Paul and Father Dom, and ghosts themselves). CeeCee has a mad crush on Adam McTavish, another of Suze's Carmel friends, whom CeeCee has known since grade school. CeeCee and Adam become her closest friends in California and often unknowingly help Suze with her mediations.
In the second half of the series, Suze meets someone who drastically changes her life: a fellow mediator named Paul Slater. His techniques and intentions differ greatly from those of Suze and Father Dominic, however. Paul is cunning, selfish, and more often than not, contemptible. Upon realizing that she is a mediator, he quickly becomes obsessed with Suze, determined to turn her feelings away from Jesse and towards himself, even if it means Jesse's destruction.
However, Paul also teaches Suze a lot about her self. Through Paul, she learns that she is more than a mediator: she is a shifter, someone who can not only speak to and touch the dead, but can travel to the other world, and can go back in time and change history. And when Paul decides to break Suze and Jesse up once and for all by traveling back in time to prevent Jesse's death (and thereby prevent Suze from ever meeting him), Paul inadvertently does the one thing that can lead to a happily-ever-after for Jesse and Suze. Thus, despite his completely repulsive personality, he turns out to be the most important person in Suze's life after Jesse.