She was a heroine when she first joined the show, but she began a long, slow & deep dissent into total villainey (her true DiMera roots surfaces), by the end of her run she was a cold/heartless/jealous/vengeful/cruel villainess.
Here is a little discription on Kristen's backstory:
Kristen initially came to Salem in 1993, as a social worker who soon met up with John Black. John couldn't understand why Kristen wanted nothing to do with him until she revealed the shocking truth -- she was Stefano's foster daughter and had been raised by Stefano DiMera after the death of her parents when she was a child. Kristen and John were eventually able to overcome her connection to Stefano, but they weren't able to overcome the fact that she was engaged to marry Stefano's son, Tony. Unbeknownst to Kristen, she had actually become engaged to André DiMera who was pretending to be Tony DiMera. The real Tony was being held on an island somewhere in the South Pacific.
After numerous manipulations, Kristen wound up marrying André believing he was Tony, but it wasn't long before she and John became lovers again, which was only helped by André having gone temporarily blind. Kristen eventually left André and prepared to have a future with John when the two learned he was a Catholic priest. Kristen could not come between John and his vows and returned to André, but when John was released from his vows, Kristen decided to petition the Church for an annulment so that her marriage to John would be recognised by the Church. Kristen and John were preparing for their future together when they, and many other Salemnites, traveled to Aremid. André staged his own death, framing John in the process. John was sentenced to death, with Kristen believing he did it. When John was exonerated, Kristen was relieved, even though she hadn't trusted him.
Shortly after John was exonerated, Kristen discovered a letter he wrote vowing his love to Marlena, and also discovered that the mysterious Woman in White was actually Rachel Blake, the mother she believed had died years earlier. She also learned that she was pregnant. Marlena agreed to stay away from John because Kristen's pregnancy was very delicate, and Kristen began to scheme to keep John and Marlena apart, even joining forces with Stefano in the hopes that he could get Marlena out of the picture permanently. Stefano held Marlena captive, and after John helped in her rescue, Kristen lost her baby, but convinced the doctor to keep quiet. Stefano, to help her, found Susan Banks, a pregnant Kristen-look-a-like, and arranged for Kristen to adopt her unborn child when it was born. The baby, named John Black Jr., was born and handed over to Kristen and John. Once Marlena realised the truth about Susan, who Kristen had to claim was a nanny to cover up when John caught them together, she arranged to have Marlena held hostage in a secret room.
Kristen wound up in the secret room with Marlena after Susan turned the tables on her, but luckily the two were rescued after Laura Horton forced Susan, who was about to marry John, to admit everything. Susan got her son back, and Kristen was arrested, but soon released on bail. John and Marlena began moving on with their lives together, so in a last-ditch effort to keep them apart, Kristen located a dying Roman. Because Roman could not handle any stress, John had to pretend to be married to Kristen while Marlena pretended that Roman was the man she wanted. She attempted to get close to the baby, whom Susan had rechristened Elvis (aka EJ DiMera), but her plans failed, and Stefano turned his back on her. John and Marlena later got Susan and her son out of the country. Kristen then kidnapped Susan's sister, Sister Mary Moira, and used that to bring Susan to town. When Susan didn't bring the baby, she sold her to an island harem. She had to pretend to be Susan when her boyfriend, Edmund, and his mother showed up in Salem, and even married him during her pretending. She had no choice by then, as Salem assumed that Kristen had died -- a body that looked a lot like hers was found in a swimming pool.
Susan and Edmund were able to meet up again, and Susan confessed that the body, whom Edmund had accidentally killed, believing she was Kristen, was also her sister -- Penelope Kent. She was dying, so Edmund couldn't be held at fault. Susan arranged to switch places with Kristen, leaving her in the island harem.
Hope that helps :)