Isabella Marie Swan
Bella, as she's more commonly referred to, is an average teenager whose parents are divorced. When her mother, Rene, gets married to Phil, a minor league baseball player, Bella decides to leave Phoenix, Arizona and go live with her dad Charlie in the town of Forks to make things easier for the newly married couple. At Forks High School, Bella's intrigued by the five mysterious Cullens. She's most intrigued by the extremely beautiful Edward Cullen, whom she will eventually end up falling in love with.
Edward Anthony Masen Cullen
Edward's parents passed away with influenza in 1918 - when Edward was just 17. Carlisle, who later becomes his adoptive father, saves Edward from dying by turning him into a vampire. Edward has the supernatural ability to read minds, and what intrigues him about Bella is the fact that can't read her mind at all. The scent of her blood drives him crazy, even though the Cullens have a morality in which they don't consider humans as food. Edward knows that being with Bella is dangerous for him, but Bella refuses to let go of him and eventually, their friendship blossoms into eternal love.
Jacob, a native of the Native American tribe of Quileute, is a werewolf who falls in love with Bella in the sequel to Twilight - New Moon. Bella met Jacob on her visit to Jacob's town of La Push, and that's where Jacob told her of a legend which leads him to believe that the Cullens are vampires. The vampires and werewolves are mortal enemies, thus it's hard for Bella to stay friends with Jacob. According to a treaty between the vampires and werewolves, if Edward chooses to fulfill Bella's wish of turning her into a vampire, there will be a war between the werewolves and vampires. Jacob is overconfident of the fact that Bella loves him more than she loves Edward.
*More to be added!*