The friendship between Fiona and Katie formed in Underneath It All, but ended abruptly when Katie and her best friend Marisol Lewis pressured and tricked Fiona into taking the blame for a prank gone wrong carried out by the grade 12 students at the beginning of the year.
Season 11In Don't Panic (2), Fiona is seen debating whether to vote for Katie, Mo or Marisol, calling Katie a liar.
In Underneath It All, Fiona sets her sights on being friends with Katie and Marisol. In class, Katie introduces herself to Fiona, and Fiona invites her and Marisol to a party she throws, which they agree to go to. When the three are walking in the halls saying that the guest list must be exclusive, Fiona suggests Imogen to come, they don't like the idea, but then think it's not so bad after all. At the party, when Katie offers Fiona a chair in the social committee, Imogen comes out in a pretty black dress. Fiona thinks she looks gorgeous, but Katie gets jealous when her boyfriend Drew agrees with Fiona. When playing Truth or Dare and Fiona is dared to chug tequila, Imogen blurts out that Fiona's an alcoholic, and Katie questions this, but Fiona denies it. At Degrassi, Katie has her key to let in all the seniors for a prank by covering the school in tin foil. When Imogen and Marisol go down the halls of Degrassi, Marisol returns without Imogen, Katie tells Fiona and Marisol to just come to avoid trouble. The next day, Katie and Marisol convince Fiona to tell Imogen to cover keep quiet about the prank. When she returns to tell them, Fiona says they should just come clean, but Katie denies this. Fiona, upset for treating Imogen badly, tells Marisol and Katie that she actually is an alcoholic but is recovering. When Fiona tells them, Imogen got suspended. Fiona once again says they should come clean. to which Katie worries about, and Katie somehow agrees since Marisol forces her. In Principal Simpson's office, Fiona begins to tell the truth, but Marisol and Katie cover it up with a lie to get Fiona in trouble. Later, Fiona yells at Katie but Katie tries apologizing and hopes they are still friends and offers her a chair in the social committee, which makes Fiona angrier and blurt out they were never friends and friends don't get other friends in trouble. Fiona walks out in disgust.
In Can't Tell Me Nothing (2), Fiona confronts Katie for stealing her coffee house idea, and calls her a lazy loser.
In In The Cold, Cold Night (1), Fiona is gossiping to Imogen about Katie being in rehab for her drug problem and tells Imogen that now is the time to strike. She calls the "cheerleader-jock" student council useless and they plan to take over.
In Got Your Money (1), Katie is officially removed from her presidential status after her sabotage on Drew in the last episode, bumping Marisol up to President and Fiona up to Vice President. Marisol warns Fiona that without Katie it will be a lot of work just the two of them, and Fiona says she will be the brains Katie was. The girls then talk about the upcoming dance and running student council together.
In Sabotage (2), Fiona is at the mall telling Drew that a party will be at the loft for Clare's birthday and Jake and his new girlfriend Katie will be there which he is fine with. Katie attends Clare's party at Fiona's condo, and Fiona breaks up the fight Katie among others participate in.
In Building a Mystery (1), Katie walks by as her former enemies Fiona and Bianca are conversing. She makes dead eye-contact with both girls and gives them "the evil eye" while strutting past them. The girls then talk about their past conflicts with the overachiever.
In Degrassi: Las Vegas, she is seen helping Drew and Bianca prepare for their wedding ceremony. She and Imogen later help Bianca try on her wedding dress before they take off for Vegas. On the plane, Fiona and Imogen tell Drew what Bianca's wedding vows are. When Drew finds out that Katie, Marisol, Jake, and Mo are also going to Vegas, she agrees with Imogen that it'll be a bumpy ride considering the bad blood between the groups. Katie sees Fiona, Bianca, and Imogen and is annoyed, but agrees to be civil.
In The Time of My Life, Katie walks into prom and makes a comment saying that Fiona really outdid herself. Katie is later seen clapping for Fiona when she gets called up to deliver her valedictorian speech.
- They have both been sent to rehab.
- They were both love interests of both Torres brothers, although Fiona was more interested in Adam, while Katie was interested in Drew.
- Katie also had a conflict with Fiona's ex-girlfriend Imogen while Fiona also had a conflict with Katie's best friend Marisol though both conflicts faded out.
- They both had a spring fling.
- Fiona was roommates with Katie's ex-boyfriend Drew.
- Fiona planned the wedding of two of Katie's enemies, Drew and Bianca.
- They both were at Drew and Bianca's "wedding" in Vegas.
- Katie shares a lot of similarities to Fiona's best friend Holly J. Sinclair:
- They both were on Student Council.
- Mike Dallas had a crush on both girls, although neither of them dated him.
- They both graduated in the Class of 2013.