Destroyed Bridget Forrester's marriage by sleeping with her husband, Nick Marone 
Fainted on purpose to stop her sister Brooke Logan's wedding to Ridge Forrester 
Told Bill that her fiancé, Ridge, had kissed Caroline Spencer, knowing Bill would attack Ridge 
Slapped Brooke Logan 
The youngest child of Stephen and Beth Logan, teenaged Katie felt as if she were in the shadow of older sisters Brooke and Donna Logan, which, along with a bad case of acne, made her feel insecure. Katie brightened when Rocco Carner asked her out; Katie considered Rocco her boyfriend and couldn't see that Rocco was more interested in Donna. Katie was furious when she caught Donna and Rocco kissing. Katie forgave them, but Rocco wanted to marry Donna, and put an engagement ring in Katie's coat pocket, thinking the coat was Donna's. Katie found the ring and assumed it was for her. Katie was hurt despite Rocco letting her down easy, but soon she was able to move on with Kurt, a college classmate.
Katie was thrilled when an "anonymous" tip (really dropped by Stephanie Forrester, who wanted to lure Beth away from her husband, Eric Forrester) indicated that Stephen, who had walked out on the Logans years before, was in Arizona. After her parents reconciled and moved to Paris, Katie faded into the background for years, occasionally seen working at the Insomnia Café or when Brooke needed a babysitter.
In 2007, Katie arrived for Donna's wedding to Thorne Forrester. But Katie overheard Donna admitting she was only marrying Thorne to get back at Stephanie for hurting Brooke; Katie went to Thorne, and the wedding was called off. Having become skilled in public relations, Katie decided to stay in Los Angeles when Forrester Creations offered her a position.
Katie's brother, Storm Logan, shot Stephanie and framed Stephen for the crime. After it was discovered that Storm pulled the trigger, Katie convinced Stephanie not to press charges. But when the unstable Storm took a gun to the home of Ashley Abbott, who had broken up with him, Katie followed him there, and Storm's gun went off in the struggle. Critically wounded and in need of a heart transplant, Katie was devastated when Storm shot himself in order to donate his heart to her. But when Katie's body rejected Storm's heart, her doctor niece, Bridget Forrester, worked on possible treatments.
A dying Katie was moved when Bridget and her fiancé Nick Marone threw her the high school prom she never had; alone with Nick on Catalina Island, Katie fell in love with him. Once Katie recovered, she officiated at Nick and Bridget's wedding; soon it was revealed that Katie had made love to Nick off-screen as a dying wish, and the encounter left her pregnant. Katie and Nick prepared for their baby after Bridget walked out, but Katie miscarried. After briefly fixating on Nick's son, Jack, Katie bounced back in time for her wedding to Nick, where Nick's mother, Jackie Marone, joined Bridget in making Katie feel so bad for betraying Bridget that Katie postponed the ceremony.
Seeing that Nick was still drawn to Bridget, Katie got drunk and met womanizer Bill Spencer, Jr. Katie appreciated that Bill didn't take advantage of her in her impaired state. Katie and Bill began to date, and Katie was touched when the ruthless tycoon admitted no one else had gotten past his armor. But one day Katie found a DVD left by his late father, Bill Spencer, Sr. which instructed Bill to avenge him by taking over Forrester Creations. When Katie questioned Bill's reasons for seeing her, he declared his love and his intention to give survivor Katie a chance to do something important. Katie was floored when Bill proposed to her, then took over Forrester and made Katie its CEO. With her parents at her side, Katie joyfully married Bill in her old neighborhood of Sherman Oaks.
Katie struggled to keep the peace when she put Donna and Brooke in the leadership positions the Forrester men refused. Katie stepped down when Bill suddenly returned the company to the Forresters; she never found out that Bill had only done so because the young Steffy Forrester had blackmailed him over his attraction to her. When Bill discovered that Liam Cooper was his son, Katie encouraged a father-son relationship between them, but soon saw a darker side to Bill when he plotted to kill Amber Moore, who claimed Liam was her baby's father. Detesting violence because of what happened with Storm, Katie walked out and refused to return until Bill agreed to therapy with psychiatrist Taylor Hayes.
Seemingly working out their problems off-screen, Katie surprised Bill with a vow renewal ceremony, only to find out that Bill intended to leave her for Steffy. Katie had a heart attack; from her hospital bed, Katie demanded that Bill choose between her and Steffy. Katie kept her husband but fumed when Bill gave Steffy a seat on the Spencer board, and chafed when Steffy, who had since fallen in love with Liam, was included during a family vacation to Cabo San Lucas. Katie discovered that the seemingly life-threatening blood clot Steffy sustained during an ATV accident was a lie engineered by Bill, who faked Steffy's MRI to keep Liam from returning to his first love, Katie's niece Hope Logan. Katie found herself locked in a tower by Bill when she tried to tell Liam the truth. Katie escaped and turned her back on Bill, though the Spencers once again resolved things off-screen.
Katie risked a pregnancy given her heart condition, despite Bill pleading with her to have an abortion. When Katie heard that Bill sprung Hope's father, Deacon Sharpe, from jail to stop Liam's wedding to Hope, Katie went into premature labor and had a near-death experience in which she saw Storm. Afraid she'd eventually leave baby Will motherless, Katie slid into postpartum depression and took off, trying to draw Bill and Brooke together so Will would have parents. Katie recovered with the help of Taylor and Donna, but by then Katie noticed an attraction had developed between her husband and sister.
Katie forgave Bill and Brooke for kissing, but took off her wedding ring when she caught Bill in Brooke's bed recovering from a drunk-driving accident they'd kept from her. Katie, who had been neglecting to take her anti-rejection medication, went into heart failure and briefly lapsed into a coma. Katie took Bill back anyway, then became an unlikely confidante to former rival Steffy, who had miscarried Liam's baby. Later, at Brooke's birthday party, Katie raged when Taylor arrived with proof that Bill and Brooke had been having a full-blown affair. Katie asked for 1% of Spencer Publications in a divorce settlement, then gave it to Bill's sister, Karen Spencer, who owned 50%, making Karen the majority stockholder; Karen made Katie CEO of the company and fired Bill.
Katie tasked Brooke with babysitting Will, installing hidden cameras to ensure that Bill and Brooke behaved. Vulnerable after Bill nearly fell to his death mountain climbing in Aspen, Katie allowed herself to be seduced by Bill and signed papers returning control of Spencer to him. Katie steamed when she learned Bill tricked her, but was impressed when Brooke replaced Bill's papers with a pile of leaves. Later, with the company faltering under Katie's leadership, she was forced to relinquish it to Bill when Brooke produced the papers she had supposedly burned.
Katie fell for Brooke's on-again-off-again love, Ridge Forrester, after commiserating with him about Brooke's affair with Bill. With Ridge poised to reunite with Brooke, Katie pretended to faint to stop their wedding, then confessed her feelings. Though Katie's relationship with Ridge drew opposition from both their families, Katie accepted Ridge's proposal. However, Katie was horrified when Ridge flew to Abu Dhabi to keep Brooke from marrying Bill only to go missing after falling out of a helicopter into the Persian Gulf. Katie joined Eric and Thorne in the search, overjoyed to find Ridge, who survived with an impaired memory. When Ridge held Bill to blame, Katie investigated with Deacon and learned that Bill's helicopter had indeed been in Abu Dhabi that day, forcing Bill to confess.
Katie kept Ridge's confidence when his brain damage left him unable to design but didn't like it when it turned out he was able to draw by guiding the hand of fledgling designer Caroline Spencer. Learning Ridge and Caroline had kissed, Katie reported it to Bill, smiling to Will that "Daddy would take care of it." When Ridge came home bloodied by Bill, Katie told Ridge she wanted all of him, but ultimately called off their engagement. Katie resisted overtures from Bill, who made Katie a vice president at Spencer. Moving past her misgivings, Katie remarried Bill and reached out to Brooke, who made drunken asides during the ceremony. Katie supported Brooke as she went into Alcoholics Anonymous. Later, Katie was disgusted when Bill published an exposé on transgender model Maya Avant in an attempt to have Maya's boyfriend, a tyrannical Rick Forrester, removed from Forrester's CEO chair.
Katie was on hand to support Liam's half-brother, Wyatt Spencer, who wanted to start a new fashion house that Bill refused to bankroll, and Ridge, who was conflicted over having a baby with Caroline. Seeing that recovering alcoholic Brooke spent too much time alone, Katie offered her sister a job at Spencer Publications. But when Katie overheard Brooke telling Bill he'd always be the love of her life, Katie descended into insecurity and paranoia to the point she began drinking herself. Though Bill and Brooke swore nothing was happening between them, Katie continued accusing them and hid bottles in the kitchen despite a session with her therapist, Dr. Hayden, during which Katie confessed she'd stopped taking her antidepressants.
Katie got caught in a cycle of swearing off booze then drinking more heavily. Indulging at a restaurant, Katie saw Ridge in heavy conversation with Dr. Wolin, a fertility specialist who told Katie that Caroline's baby, Douglas Forrester, couldn't be Ridge's. Katie got Caroline to slip that Ridge's older son Thomas Forrester was really Douglas' father; Katie shared the news with Bill but was furious he passed it along to Brooke; Katie told Bill and Brooke to be together, then apologized for her overreaction. Katie worried she was imagining everything until Brooke confirmed she and Bill had resumed their affair.
When Bill hid Will away, claiming Katie was an unstable parent, Katie was impressed with the way Brooke convinced Bill to return Will to her. Katie stopped drinking and agreed to an amicable divorce, but couldn't persuade Bill to give her his 12.5% stake in Forrester, which Ridge wanted to stop Quinn Fuller, an unstable woman prone to criminal activity that Eric had gotten involved with.