Bonnie Sheila Bennett is Elena Gilbert's best friend. She is also good friends with Caroline Forbes. In the beginning of the series, Bonnie discovers that she is born from a line of witches and that she is also a witch. Her grandmother, a witch too, helps her learn to use her powers, which causes them to create a strong bond. When her grandmother dies, Bonnie is devastated, but she keeps training and as the series develops she becomes more and more powerful. In the midst of season two, she starts a relationship with Jeremy Gilbert, Elena's younger brother. She later discovers from warlock Luca Martin that the only way to kill Klaus, the Original Vampire who wants to kill Elena, is for her to get the power of one-hundred dead witches, which she later gets. She then pretends to be dead, a scheme of Damon's, in order to trick Klaus. But when she brings Klaus to the brink of death, Elijah does not kill him, as planned, and carries him away before she could react.
In the season two finale Bonnie is able to save Jeremy from dying after she tells her dead ancestor Emily Bennett that she loves him. However bringing Jeremy back from the dead had consequences, for Jeremy is now able to see his dead ex-girlfriends Anna and Vicki Donovan whenever he thinks of them. Bonnie later finds out that Jeremy kissed Anna when the doorway to the Other Side was temporarily opened and she breaks up with him. Later on, Bonnie reconnects with her mother, Abby, who had abandoned her as a child, who helps her to kill the original vampires. However, Abby is turned into a vampire by Damon and abandons Bonnie again. In the season three finale, she places Klaus' soul into Tyler's body thus preventing him from being completely destroyed by Alaric and saving Tyler, Caroline, Damon, and Stefan from dying (they are of Klaus's bloodline). In the beginning of season four, Bonnie finds out that there is a forbidden magic, but she is forced to use it to transfer Klaus back to his original body, as a result, angry spirits attack her grandmother's ghost.
Upon meeting Professor Atticus Shane, he begins to mentor her in regaining her powers. However, without her knowledge, begins to teach her in the art of Expression; the method of drawing magic from the spirits of a massacre. It is revealed that she is a key component to finding the cure, as she was a descendant of the witch who entombed Silas, and is the only one who can open the tomb. During the course of the fourth season, she becomes closer to Jeremy again, hinting at a possible reunion between the two. However, upon finding the cure he is killed by Silas. Feeling unimaginable grief, Silas, posing as Shane, manipulates her into believing she can bring him back from the dead, but only if she brings back every supernatural creature who has died. When Bonnie drops the veil to the otherside, she is over powered by Silas, who convinces her that she cannot breathe. Her Grams appears and convinces Bonnie to snap out of it. Bonnie, unable to kill Silas, does manage to petrify Silas. Afterwards, Bonnie is determined to make Jeremy Gilbert stay and attempts to bring him back to life. Bonnie's Grams warns her not to attempt the spell, however Bonnie tries and fails. The spell ends up being too powerful and kills Bonnie. She wakes up as a spirit, though eventually she brings the veil back up, meaning that Jeremy (who she eventually resurrected), is the only one that can see her.
At some point in season five, she is resurrected but not without some consequence. Every supernatural who dies has to go through her to go to the afterlife and thus she feels the pain each supernatural felt when they died.