Mrs. Flowers, in the novels, is a witch and she owns the boarding house where Stefan is living; in the TV series she owns the bed-and-breakfast where Katherine was staying.
Meredith Sulez is one of Elena’s best friends in the books, in the show she was introduced as Meredith Fell. In the books, she is a student and girlfriend of Alaric K. Saltzman but in the series, Meredith is a doctor and has a relationship with Alaric.
Sage in the books is a male and a vampire, but in the show, Sage is a female. In the novels Sage is a close friend of Damon, in the the TV series she used to be his mentor.
In the books Bonnie’s surname is McCollough, not Bennett like in the show. In the TV series she used the surname McCollough when she was under cover to get close to Virginia St. John.