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, which, along with a bad case of acne, made her feel insecure. Donna talked her friend, Rocco Carner, into asking Katie out to boost her morale. 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. Rocco struggled with how to let Katie down easy. Once he did, Katie was hurt, but soon able to move on with Kurt, a college classmate.
Katie and Donna were thrilled when they received an "anonymous" tip that Stephen, who had walked out on the Logans years before, was in Arizona. (It was really a hint dropped by Stephanie Forrester, who wanted Stephen to lure Beth away from Stephanie's husband, Eric Forrester.) Stephen and Beth indeed reconciled, then moved to Paris. Afterwards, Katie faded into the background for years, but could occasionally be 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 that she was only marrying Thorne as a way of getting back at Stephanie for hurting Brooke. Donna ignored Katie's pleas to be honest with Thorne, so Katie went to Thorne herself, and the wedding was called off. Katie had become skilled in public relations while she was away, so when Forrester Creations offered her a PR position, Katie decided to stay in Los Angeles.
Katie's brother, Storm, shot Stephanie and framed Stephen for the crime. When it was discovered that Storm pulled the trigger, Katie convinced Stephanie not to press charges. But Storm was on a downward spiral, and 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. Katie was critically wounded and not expected to live. A remorseful Storm, who knew Katie needed a heart transplant, took his own life in order to donate his heart to her. Katie was devastated, although grateful for her brother's ultimate sacrifice.
When Katie's body rejected Storm's heart, Katie's doctor niece, Bridget Forrester, researched ways to help Katie accept the transplant. But the prognosis was grim, so Bridget convinced her family, and fiancé Nick Marone, to make Katie's last days special. This included giving Katie the high school prom she never had, and sending her to Catalina Island, where Katie fell for Nick. Bridget came through with a therapy for Katie, who recovered. Nick and Bridget got married, and Katie hid her feelings, even officiating at their wedding. A few weeks later, Katie discovered she was pregnant. It was revealed that Nick had honored Katie's dying wish on Catalina and made love to her. Bridget left Nick and Katie to have their baby together, though Nick's mother, Jackie Marone, disapproved. Soon after, Katie miscarried. She briefly dealt with it by fixating on Nick's son, Jack, but bounced back when Nick proposed.
On Katie's wedding day, Jackie and Bridget blasted Katie for her betrayal. Katie regretted destroying Bridget's marriage, and postponed the wedding. Katie noticed that Nick was still drawn to Bridget, who had left medicine and was designing with Nick. Katie moved out and got drunk at the Café Russe, where she met Bill Spencer, Jr., a womanizer who wanted to take over the financially struggling Forrester Creations. Bill chose not to take advantage of Katie in her impaired state. Katie added modeling to her résumé at Forrester, and started seeing Bill, who told her she got past his armor like no woman had. Katie questioned Bill's reasons for seeing her when she found a DVD left by his late father, Bill Spencer, Sr., which instructed Bill to get back at Eric, Stephanie, and Donna. Bill declared his love and recognized that Katie wanted to do something important, given her second chance at life. He finally acquired Forrester Creations, made Katie its CEO, and proposed to her. Katie joyfully married Bill in her old San Fernando Valley neighborhood, with Stephen and Beth at her side.
At Forrester, Katie put Donna and Brooke in leadership positions when the Forrester men refused them, and struggled to keep the peace between the Logans and the Forresters, who tried to sabotage the company with an ugly fashion line. Meanwhile, Bill's wandering eye turned to young Steffy Forrester, who had kissed Bill before his wedding. Katie never knew that the reason Bill put the company back in Forrester hands was because Steffy had gotten a hold of Katie's engagement ring in Bill's bedroom and used it to blackmail him. Later, when Bill discovered that Liam Cooper was his son, Katie encouraged a father-son relationship between them.
Katie discovered another 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. After a few sessions with Dr. Taylor Hayes, it appeared that Katie and Bill had worked their differences out off-screen. Later, Katie planned a surprise vow renewal ceremony for Bill, only to be told by Taylor that Bill intended to leave Katie for Steffy. Katie had a heart attack while confronting Bill; in the hospital, Katie demanded that Bill choose between her and Steffy. Bill chose Katie, but Katie was enraged when Bill still gave Steffy a seat on the Spencer board.
Katie chafed during a "family vacation" to Cabo San Lucas, which included Steffy, who had fallen in love with Liam. After Steffy was injured in a ATV accident, she was diagnosed with a blood clot that would become life-threatening under stress; knowing Liam wanted to leave Steffy, Katie did some digging and learned that Bill had faked Steffy's MRI to keep Liam from his first love, Katie's niece Hope Logan. Katie was ready to tell Liam, but Bill locked Katie in a tower to prevent her from exposing him. Katie escaped and turned her back on Bill, though they again seemed to resolve their issues off-screen.
Katie soon learned she was pregnant, but Bill wanted her to abort because of her heart condition. Katie took the risk anyway, and months later discovered that Bill had Hope's father, Deacon Sharpe, broken out of jail for the express purpose of stopping Liam's wedding to Hope. Katie went into premature labor and nearly died after giving birth to baby Will; during her near-death experience, Katie saw Storm, who helped her return to the land of the living. But Katie slid into postpartum depression and was afraid she'd leave Will motherless, so she attempted to draw Bill and Brooke together. Once Donna and Taylor helped Katie snap out of her gloom, Katie realized an attraction had developed between her husband and sister. Katie learned they kissed but forgave them, though she worried about Bill's excessive drinking and the lack of intimacy between her and Bill.
Later, Katie found Bill in Brooke's bed after a drunk-driving accident they had kept from her. Katie took off her wedding ring and walked out, then went into heart failure and fell into a coma after neglecting to take her anti-rejection medication. When Katie woke up, she took Bill back, and found herself in the unusual role of confidante to Steffy, who had miscarried Liam's baby. Noticing Brooke hadn't been herself, Katie threw her sister a birthday party to cheer her up, though it was interrupted by Taylor, who arrived with proof that Brooke had slept with Bill and miscarried his baby. Katie divorced Bill, asking for 1% of Spencer Publications, which she then added to the 50% owned by Bill's half-sister, Karen Spencer; now the majority stockholder, Karen made Katie CEO and fired Bill!
Though estranged from Brooke, Katie asked her sister to babysit Will, which was really a ploy to install hidden cameras and monitor Brooke and Bill, who behaved. After Bill nearly fell to his death mountain climbing in Aspen, Katie agreed to sign papers that returned control of Spencer Publications to Bill and gave him joint custody of Will. Once Katie learned Bill had tricked her, she was surprised to find Brooke had replaced Bill's papers with a pile of leaves.
Brooke's on-again-off-again true love, Ridge Forrester, came home to reunite with her after living in Paris for a year. But when Ridge found out how Brooke's relationship with Bill had hurt Katie, Ridge approached Katie with an understanding ear. Katie fell for Ridge and stopped his wedding to Brooke by pretending to faint. Katie and Ridge faced opposition from Donna, Brooke, and Eric, who made Ridge and Brooke co-vice presidents at Forrester to push them back together. As Spencer Publications seemed to falter under Katie's leadership, Brooke, who had resumed a relationship with Bill, produced the papers she had supposedly burned and forced Katie into reinstating Bill as CEO.
Katie happily accepted Ridge's proposal, but Ridge didn't want Brooke marrying Bill and followed them to Abu Dhabi to stop the ceremony. Ridge fell out of a helicopter into the Persian Gulf and went missing; Katie, Eric, and Thorne flew to the Middle East to join in the search. Ridge returned with an impaired memory, and Katie was concerned when he had trouble remembering their engagement. When Ridge insisted Bill was responsible for his accident, Katie teamed with Deacon and discovered that Bill's helicopter had indeed been in Abu Dhabi that fateful day; this information scuppered Bill and Brooke's attempt at a reconciliation.
Katie was alarmed when Ridge revealed his brain damage left him unable to design, but agreed to keep it secret. Later, Forrester model Maya Avant told Katie that Ridge was spending a lot of time working with Caroline Spencer; Ridge reported that he was able to draw by guiding Caroline's hand. When Katie learned that Ridge and Caroline had kissed, she took a trip to Dallas to visit her father. Upon returning, Katie developed a new friendship with Bill and told him about Ridge and Caroline's involvement, privately smiling to Will that "Daddy would take care of it" -- after Ridge came home bloodied by Bill, Katie told Ridge that she wanted all of him, to which Ridge agreed. However, Katie ultimately called off their engagement and became intrigued by Bill, who made Katie vice president at Spencer and otherwise tried to woo Katie into coming back to him.
Despite her initial misgivings based on their past, Katie agreed to remarry Bill. Hoping to heal the family, Katie asked Brooke to be matron of honor but was unnerved by Brooke's drunken asides during the ceremony. Katie supported Brooke when she went into Alcoholics Anonymous, but disapproved of the assistance Brooke got from Deacon, a fellow addict. Later, Katie watched as a battle ensued at Forrester between tyrannical CEO Rick and the team of Liam and Ridge, who wanted to take over the company. When it was discovered that Rick's girlfriend, Maya, was transgender, Katie thought she talked Bill out of publishing the story but was disgusted when Bill publicly exposed Maya anyway.
Katie supported the idea of Liam's half-brother, Wyatt Spencer, starting up a new fashion house with his girlfriend, Ivy Forrester, but Bill refused to bankroll it. Katie also offered counsel to Ridge, who was in disagreement with Caroline over whether to start a family. Later, Katie worried that Brooke was alone too much, so Katie offered Brooke a job at Spencer Publications. But when Katie overheard Brooke telling Bill he would always be the love of her life, Katie disowned Brooke and told Bill she would never let Brooke have him.
Katie began drinking heavily to cope with her relentless fears, worsening after learning Brooke kept a photo album dedicated to Bill. Katie insisted to Bill she didn't have a drinking problem, but after little Will couldn't rouse her, she privately put in a call to her therapist, Dr. Hayden, and confessed she'd stopped taking her antidepressants. Katie caught Brooke and Bill discussing Katie's drinking and accused Brooke of trying a new angle to seduce Bill.
Katie had a meltdown in front of Brooke, privately sipping whiskey from a broken glass after promising her sister she would stop drinking. During a dinner with Bill and Brooke, Katie had a hallucination they were flirting and freaked out about it, causing Bill to ask Katie if she was still abusing alcohol. Katie denied it but kept a stash hidden in the kitchen, which Brooke found. Katie begged Brooke to keep it a secret from Bill, lashing out when Brooke exposed her.
Katie drank secretly at a restaurant and saw Ridge in heavy conversation with Dr. Wolin, who introduced himself to Katie as Ridge's fertility specialist and revealed that Ridge could not have given his wife, Caroline, a baby. Katie probed Ridge but got nowhere; later, Katie clammed up when Rick asked into Ridge and Katie's presence at the restaurant and announced that Dr. Wolin had died after being hit by a truck.