Steffy as a child...Steffy and her twin sister Phoebe were born onscreen on September 21, 1999, to Ridge Forrester (Ron Moss) and Dr. Taylor Forrester (Hunter Tylo). Taylor nearly died giving birth to them after suffering from tuberculosis.
Steffy was named after her grandmother, Stephanie Forrester (Susan Flannery). In February 2001, Ridge and Taylor took the twins and their older brother Thomas Forrester to Saint Thomas, where they renewed their wedding vows. The family, along with the babysitter Catherine, then took a boat trip, and two-year-old Steffy fell overboard during stormy weather and was presumed eaten by sharks.
However, Steffy turned up weeks later at Morgan DeWitt's house. Morgan, an old flame of Ridge's who desperately wanted to have his child, had kidnapped her and was holding her captive. Taylor grew suspicious of Morgan and was shocked to discover Steffy in her house. As Taylor was about to take her daughter home, Morgan returned and held them both captive. Ridge eventually saved his wife and child by driving his car through Morgan's house as she was about to shoot Taylor. Morgan was eventually committed to a psychiatric institution.
Family problems continued to spiral when Taylor was presumed dead after being shot by Sheila Carter (Kimberlin Brown), Steffy and her siblings wanted Bridget Forrester to live with them and raise them. Instead, Ridge married Brooke Logan a woman he had been involved with and engaged to prior to meeting Taylor.
A new addition, R.J Forrester was added to their family when Brooke and Ridge had their first child together before Ridge and Brooke married. Years later, Taylor was revealed to be alive and Ridge and Taylor renewed vows. The following year, they split up due to Taylor revealing the truth about her sleeping with a man named James in 1993. Steffy left town for schooling in England.
Steffy as an adult...Steffy returns from college and begins working at Forrester Creations. She meets Marcus Walton (Texas Battle) and digs in his past, finding out that he is the son of Donna Logan (Jennifer Gareis). She begins dating Marcus but the relationship doesn't work out because of the rivalry between the Forresters and Logans.
Phoebe is killed in a car accident resulting from an argument with Phoebe's ex-boyfriend, Rick Forrester (Kyle Lowder). Steffy later begins a romance with Rick, much to her family's disapproval; they eventually broke up after it was revealed Rick was getting back at Ridge because of a past grudge.
Steffy pretends to seduce Bill Spencer, Jr. (Don Diamont) and wins back Forrester Creations from him. She also makes sure that the new Forrester Creations would exclude Brooke, Donna and Katie. Steffy takes interest in Brooke's daughter Hope Logan's (Kimberly Matula) boyfriend, Oliver (Zack Conroy) but fails to win him over. Steffy makes it up to Hope by making a video for Hope's graduation party. Instead of bringing them closer, the video revealed Brooke's sleeping with Oliver (thinking he was Ridge) and intensified the feud between the Logans and Forresters. Steffy says the video was tampered with, but no one believes her and she is fired from Forrester Creations.
After she was fired, Taylor signed over her 25% stock to Steffy which brought her back into the company. It was later revealed the tribute video was tampered with by Liam Spencer (Scott Clifton), Bill's son. Steffy forgives Liam, and develops romantic feelings for him; however, he is with Hope now. She fails to win over Liam and leaves for Paris, France for an extended business trip.
Steffy returns from Paris with her sights set on winning over Bill; who is married to Katie Logan (Heather Tom). When Bill nearly drives Amber Moore (Adrienne Frantz) to her death at a cliff, Steffy (who saved Amber) keeps this a secret, unlike Bill's wife Katie who goes crazy. Bill and Steffy has an affair but it ends when Katie has a heart attack after learning of it.
Steffy then falls in love with Hope's fiancé Liam, who saved her from drowning. While Hope it too busy with work, Steffy is there for Liam. When Hope sees them kissing she breaks up with Liam, who instantly proposes to Steffy and they are married in Aspen, Colorado, despite Hope nearly breaking up the wedding. Liam was still in love with Hope at the same time.
During a trip to Cabo San Lucas, Mexico for Forrester Creations and Spencer Publications, Steffy attempts to keep Liam's sights away from Hope, who is on the same trip with Thomas. Liam catches Hope and races after her, with Steffy following him. Steffy later ends up in a road bike accident and her family is alerted at the hospital that she has a clot in her brain that could be fatal. Liam remains with Steffy but breaks up with her when it was found out Bill faked the blood clot news.
Steffy and Liam decide to divorce, giving him six months to decide if he truly wants Hope or not. Steffy later sees the love he had for Hope, and signs annulment papers, which Liam ends up tearing up feeling nostalgic about their marriage. However, he goes ahead with the divorce and marries Hope in Italy. Prior to their wedding, Liam confessed his love for Steffy while under the impression that Hope had called things off.
Before Hope and Liam's official legal wedding in the United States, she spends a night on the town with Liam. Hope left Liam for good, and he reunited with Steffy. However, Hope finds out that their Italy wedding the previous year was sabotaged and wants Liam back. Steffy discovers that she is pregnant with Liam's child. She decides to leave for Paris to visit Ridge and figure things out, and tells Liam to spend time with Hope.