December 11, 1991 to August 1994 [contract];

April 28, 2009 to April 29, 2009; November 12, 2009 to November 13, 2009; July 22, 2011 to July 26, 2011; March 28, 2012 to July 2, 2012; January 4, 2013 to January 8, 2013; July 10, 2013 to July 16, 2013; April 16, 2014 to April 18, 2014 [recurring]

Karen Spencer is the daughter of publishing magnate Bill Spencer, Sr., but she was kidnapped as a baby and spent the bulk of her childhood growing up as Faith Roberts on a farm near Boise, Idaho; she moved to Starlight, Texas with Bonnie Roberts, the woman "Faith" believed to be her mother. "Faith" never would have known about Bill or her identical twin sister, Caroline Spencer, had Blake Hayes not walked into her diner one day; Blake pined for his ex-wife, Taylor Hayes, who was now married to Forrester Creations heir Ridge Forrester. Karen was shocked to learn her true identity from Blake, who explained that Ridge had been widowed by Caroline when she died of leukemia. Karen allowed Blake to take her back to her hometown of Los Angeles and dye her hair to match Caroline's, not knowing Blake planned to use Karen to lure Ridge away from Taylor.

Karen became a bonafide Spencer when Bill matched her fingerprints to those taken from her crib as an infant. Karen met Ridge, who was moved to see Caroline's double, but when Ridge only wanted Karen as a friend, Karen was bound and gagged by an angry Blake. Karen managed to get to a phone and was rescued by Bill with the help of Ridge's brother, Thorne Forrester, who also had been married to Caroline. After Blake left town in defeat, Karen and Thorne began dating, but Thorne's estranged wife, Macy Alexander, wanted to reconcile; the trio tried living under the same roof, but the Three's Company arrangement soon devolved into a foodfight between Karen and Macy.

Karen won out over Macy when Thorne mistakenly believed Macy had been carrying on with tennis instructor Jake Maclaine. Karen lost her virginity to Thorne and took on a leadership role at the ailing Spectra Fashions, in which Bill had invested a significant amount of money. Karen enjoyed getting to know owner Sally Spectra and her ragtag employees; Karen helped get romance novel cover boy Fabio to appear at Sally's bachelorette party.

After Thorne and Macy reunited, Karen became friends with Forrester Creations CEO Brooke Logan and her attorney Connor Davis. Karen had to prove she and Connor were only friends when Brooke's liaison, Sheila Carter, presented Brooke with a photo of Karen and Connor in a clinch. Later, when Karen and Connor did decide to date, they were invited to a pool party with Macy and her new beau, Anthony Armando, who duped Karen and Macy into wearing bikinis that became transparent when wet; however, Karen and Macy got the last laugh when Spectra employee Keith Anderson gave Anthony and Connor transparent swim trunks instead.

Later, Karen asked Connor to take an HIV test but still held off making love to him until one night when she had too much to drink. When Macy admitted overhearing he'd gotten Karen drunk on purpose and didn't use a condom, Karen taught him a lesson by pretending to be pregnant; Karen got Connor down the aisle before she finally revealed the truth. Karen was upset when Connor delayed signing their marriage certificate and dated Darla Einstein, but ultimately Karen and Connor parted friends, and Karen went back to her life in Starlight.

Fifteen years later, in 2009, it was revealed that Karen had taken a high-powered position at Spencer Publications' New York office; Karen visited Los Angeles upon the death of her father and was surprised to learn she had inherited a 50% share in the company, since Bill had been old-fashioned about installing women in his boardroom. Karen attended the wedding of her half-brother, Bill Spencer, Jr. and Katie Logan, coming back for their surprise vow renewal ceremony in 2011.

But Karen had her own surprises: a secret daughter by sperm donor whom she named after her late twin sister, and the fact that she had come out as a lesbian and married her partner, Danielle. When young Caroline was invited to work at Forrester Creations in 2012, Karen and Danielle followed her to Los Angeles, where Karen was adamant about keeping her marriage a secret from Caroline's new beau, Thomas Forrester, and especially her homophobic brother Bill. With support from Caroline and Thomas, Karen finally told Bill, relieved when Bill ultimately accepted Danielle into the family.

Knowing Bill's temper, Karen wrongly suspected he had pushed Caroline over a balcony during an argument. Later, Karen took part in an intervention for Bill, who was notorious for drinking too much. Karen was shocked to learn that Bill had committed more than one crime to keep his son, Liam Spencer, from marrying Katie's niece, Hope Logan. After Bill had an affair with Katie's sister, Brooke Logan, Karen became Spencer's majority stockholder when Katie gave her the 1% share she obtained in her divorce; Karen used her power to remove Bill as CEO and put Katie in his place. Karen refused to reinstate Bill, even when the company began faltering under Katie's leadership, but, forced to honor papers Bill tricked Katie into signing, Karen reverted to being an equal owner with Bill and allowed Bill to be CEO again.

Karen hasn't been seen since 2014, but off-screen she nursed Caroline back to health when she was hit by a car, and later got a chance to meet her grandson, Douglas Forrester, when Caroline came to New York for a visit. Karen and Danielle also helped Caroline raise Douglas when she moved home after things didn't work out with Douglas' father, Thomas.