Isabella has a big crush on Phineas. Phineas' feelings for Isabella, though, are somewhat unclear, although his feelings seem to have gone from completely oblivious to romantic as time passes. She has many romantic daydreams about him and also believes she will one day marry him, but she gets angry if Phineas, with his obliviousness, cannot notice her hints flirting with him, which is generally whenever she acts romantically around him. She cutely greets him with the phrase "What'cha doin'?" She has even directly referred to Phineas as "The very thing she lives for." Also, Isabella revealed in the song "What'cha Doin'?" that she is always dying to come over to his house to see him and ask 'What'cha Doin'?' and that she specifically says it to Phineas and not Ferb. She does this because it is for capturing Phineas' attention. Isabella swooned for "The Beak" a few times during "The Beak's Theme Song", not realizing that it was still Phineas she was swooning for.
One example of this is when Isabella and Phineas were alone in Paris looking for parts, and Isabella was trying to be romantic, but Phineas seemed very oblivious to her. Then when the plane crash landed on the deserted island and Phineas tried to find something to fix the plane, Isabella began crying because Phineas did not even take notice of her and she would have given anything for Phineas to sit down next to her and enjoy the sunset. Then Phineas ran up next to her, having a small mental breakdown, desperately thinking of an idea to get off of the island. Figuring that none of the ideas would work, he gave up hope entirely and sat down with Isabella to enjoy the sunset with her. Seeing how broken Phineas was, Isabella realized that this was not the amazing kid she had fallen in love with, so she gave him a pep talk, and almost admitting her love for him in the process, but she quickly caught herself and said something else. She then mentioned Ferb's map and Phineas thanked her for the inspiration with a hug, leaving her in a trance. Later when they make it back to Danville, Isabella hugs Phineas for regaining his confidence to achieve his goal. Not even 30 seconds later they sing a duet with Phineas taking her hand several times and also showing several signs that he returns her feelings for him.
However, when Phineas constructs a device to find cute things to search for Meap, he says he had taken Isabella's cuteness into account and adjusted the settings accordingly. When he puts the settings back to normal, the device overloads from an excessively strong cute signal from her. She uses her position as Fireside Girls troop leader to stay near him and participate in his Big Ideas, under the guise of earning the "Help Thy Neighbor" patch (which she has earned at least 52 times). It is unsure whether Phineas has the same feelings towards Isabella, but mostly he seems oblivious. Phineas feels bad about ditching her, and also says that he cannot tell her he is The Beak because she is "close to him". Phineas is sometimes worried about Isabella getting hurt. Also, when Isabella tells Phineas he is brave, he replies with, "You were too!" She learns that she will be aunt to Candace's children. Although who she married was unknown, it will most likely to be Phineas or Ferb, since Phineas and Ferb are now being shown returning Isabella's feelings (although it probably will not be Ferb because in "Happy Birthday Isabella!", Ferb declined so Phineas and Isabella could get some alone time, and then Isabella fist bumped Ferb, implying thanks). She sometimes disguises her affection for them (but very rarely) like hugging him in a crowd when everyone is hugging Perry, Isabella hugs Phineas and instead of Perry.
In "The Great Indoors", Phineas helps Isabella get patches after rain spoils her plans to get them, which had made her sad. The rain was caused by Heinz Doofenshmirtz's Rain-inator. In "Canderemy", Isabella wanted Phineas to go to a sold-out Stumbleberry Finklebat movie, but Phineas interrupted her so he could finish his robodog (as seen in the short version of Today is Gonna Be a Great Day). At the end, Phineas goes with her, but with Ferb combined with him from Heinz Doofenshmirtz's Combine-inator.
While on hot air balloons, Isabella's balloon is behind Phineas's. Isabella asks him what he's doing, seeing him fidgeting with something on his balloon. He explains that his wheel is broken, but he stops talking because he heard something behind him. He turns around to see Isabella, who asked if he needs help fixing it and somehow got from her balloon to his. Phineas replied, asking her to hold the wheel still. Isabella immediately jumps on the opportunity and holds the wheel still, obviously overjoyed that she can put her arms around Phineas. Phineas did not seem to mind and replied by saying she is holding it perfectly, partially hinting that he likes having her arms around him. Isabella smiled and they stay in the same balloon together for the rest of the ride.
On Phineas' birthday, Isabella helps create a digital video for his birthday. She created some digital editing voices for herself when Phineas' quoting: "We'll be together forever Isabella" and "Isabella, will you marry me?". Later that day, Phineas stated that he enjoyed being around the people he loves. While he says this, a tear runs down Isabella. This may mean that Phineas may like her now as more than a friend.
Phineas explains that the teleport function of his and Ferb's prototype cellphone, built for Candace, will "take you to the place or person you're trying to reach." He demonstrates this by asking Isabella to stand on the driveway, to which she willingly agrees. Phineas specifies Isabella's name to the phone ("Go to: Isabella") and is transported directly beside her - so close, in fact, their noses touch. Isabella asks Phineas what he's doing ("Hey, Phineas. What'cha doin'?"), to which Phineas responds "I'm transporting!" Neither flinch, pull their noses away or appear flustered throughout the exchange. Isabella seems very pleased with the close contact - that is until Phineas transports back to the garage and she angrily replies with a "Dang it!". As the song Aerial Area Rug starts, Phineas reaches out for Isabella's hand to help her onto the flying carpet. They are also seen looking into each others' eyes, lying down close to each other, or just sitting down together away from everyone else throughout the duration of the song.
Near the end of the 2011 Disney Channel Original movie, when Isabella and several others were lined up to have their memories of witnessing Perry as a secret agent erased, Carl is going to wipe the memories of everyone who found out that Perry was a secret agent using a machine called the "amnesia-inator". Isabella asks Major Monogram if the machine would make them forget everything that happened that day, he assures her of it, and she kisses Phineas. However, Phineas does not show any affection or does not remember nor does Isabella. Which it does not count that they show they love each other or have affection. She then tells Carl to fire up the machine, much to the dissatisfaction of Phineas. Right afterward, Phineas objected to having their memories erased and proceeds to cry out "Wait, wait, WAIT", only to be ignored. This shows that he did not want to forget that moment and that he may have feelings for Isabella after all and that would also mean that Phineas would have preferred to remember the kiss than lose Perry.
In the episode "Ferb Latin", Phineas and Isabella danced and sang together while singing the "Ferb Latin" song about the new language, Ferb Latin, and he smiled at her sweetly. They also touched their shoes while singing the song. As Isabella walks towards the stool to sing "Let it Snow, Let it Snow, Let it Snow", Phineas looks at her endearingly. The song "Let it Snow, Let it Snow, Let it Snow" has a romantic nature and during the song, Isabella lightly kisses a snowman shaped like Phineas on the nose. The snowman quickly melts after she does this. She also forms a heart with her fingers around a Phineas-shaped silhouette in a window. When Phineas first sees Princess Isabella, he dreamily admires her. Then, when Phineas and Ferb come to rescue her from Doofus Khan, she thanks and hugs Phineas. After the escape, Doofus Khan finds them again and Isabella hides behind Phineas; they escape together after Doofus Khan's dragon fell on top of the giant terracotta warrior after the battle. However, these events partially do not count due to the fact that this episode is in a parallel world.
Phineas carries Isabella to safety while trying to evade threatening laser beams. Later, when Phineas becomes hypnotized by Mitch's cuteness, Isabella is the only one able to snap him out of the trance. Phineas alludes to Isabella's own cuteness as a 'scientific fact', also claiming that it 'goes right to her core'. Phineas and Isabella share a jet ski when following Candace and Crikey through Danville sewers and canals. They're constantly smiling at each other during the ride and he is holding her shoulders for support. Also, when he and Irving somehow switched places during the ride, he seemed a bit disappointed when he noticed he wasn't with Isabella anymore.
When Phineas and Ferb give Isabella a big birthday party, all she really wants is to be alone with Phineas. At the end of the party, she expresses adoration for his (-inator-assisted) grand finale: butterflies, which is what he always gives her. After the party, Phineas asked if she wanted to go out with him and Ferb for ice cream, but Ferb discreetly opts out, allowing Phineas and Isabella to go out together.
Isabella is best friends with Ferb, likely because he's the stepbrother of her crush, although she seems to get annoyed when Ferb goes with Phineas. Along with Candace, she has outright stated that Ferb is Phineas's stepbrother. Unlike her time with Phineas, Isabella shows no romantic interest in Ferb although she does like him. However, after time-traveling to the future, she learned that she might marry either Phineas or Ferb. Upon this discovery, Ferb turned around and winked at Isabella, who was stunned at the possibility that she may marry someone else other than Phineas. This may indicate that Ferb may harbor a bit of affection for Isabella, or that Ferb just fancies himself a ladies' man.
It is also possible he was simply teasing her. Ferb is slightly irritated and gets mad when Phineas gets all of Isabella's attention and praise for being a superhero, but it can be just the thought of him being the one who controls the lower half and saves Danville from the falling sign, but Phineas gets all the attention. When they were stranded on the uncharted island, Ferb listened while Isabella talked about Phineas not noticing her and offered her a handkerchief when she started crying. He knows about Isabella's crush on Phineas and he also walked away when Phineas came to leave them alone. He may not have any affection to Isabella however he knows he might get married to her. It is most likely that he does not have affection for Isabella since he has more affection for Vanessa Doofenshmirtz, though he thinks he doesn't have a chance to get Vanessa's love. Ferb has also demonstrated awareness of Isabella's affection for Phineas; after Ferb helped Phineas throw a party for Isabella when Phineas offered her ice cream with Ferb, he left to have Phineas and Isabella have some alone time together. Isabella was pleased and both bumped fists.
Isabella is friends with Baljeet. They hang out on occasion, usually when they are brought together by Phineas and Ferb's Big Ideas. She won a bet against him on who would win the fight between Buford and the squid. Though when trying to find the purpose of a machine Baljeet did jump at the chance to have a romantic dinner with Isabella, even a staged one. Isabella told him he ran at the table. This is also hinted when Baljeet asked Isabella if he made a good Phineas, she responds with "No, no you weren't."
When Phineas and Ferb built a super computer, Isabella is seen thinking about Phineas on the super computer. This makes Baljeet mad, showing he may have a crush on her. While attempting to climb Danville Mountain, Isabella shows significant enthusiasm for Baljeet's eagerness to perform the task without the help of any Flynn-Fletchers' inventions, which stands in contrast to the attitude of Ferb and, especially, Phineas.
So far, Isabella is the only character to stand up to Buford and has done so regularly. However, she showed deep sympathy when he was left emotionally broken during the loss of his goldfish and knew without a doubt he'd win in a confrontation against a giant squid.
Isabella believes without a doubt in her mind that Phineas, though she has faith in him, would be left seriously injured after a fight with Buford. Isabella has demonstrated dominance over Buford in video games, as she overpowered in a one-on-one fight and single-handedly annihilated a giant virtual video game boss version of Buford.
When Buford said her catchphrase, she elbowed him very hard in the arm but when Candace and Suzy said it she got mad but not physical showing that it's a bigger deal in her mind for whatever reason that may be. When Buford hides behind her from the giant squid, Isabella didn't even notice the squid or the fact that Phineas and Ferb are missing. She just cared that Buford found Biff. Of course, this could just be obliviousness and nothing more.
When Phineas and Ferb were explaining about the weather, Baljeet became 'hypnotized' and Isabella snapped him back to reality. When Buford also became 'hypnotized', Isabella slapped him, causing Buford to thank her. When she had a hold of someone's hand in the dark then found out it was Buford's, not Phineas', she had a negative reaction in the form of a loud "EW!".
Isabella and Candace are good friends. They both know about each other's respective crushes and share advice and experience. While in Paris, however, Isabella felt and displayed envy of the time Candace was able to spend with Jeremy.
When Isabella was delighted to know she was being called 'aunt' by Candace's future children, initially believing she'd grow up to marry Phineas, Candace responded saying there was the possibility it wouldn't be Phineas she would marry, but rather Ferb. Candace teamed up with Isabella to beat Buford and Baljeet. Isabella, Phineas, and Ferb also helped Candace get a ticket to the Paisley Sideburn Brothers Concert. Even though they like each other in general, Isabella was mad at Candace when Melissa was more interested in Candace's achievements than Isabella's, and doesn't like her when taking credit for something, yet they became reconciled when Candace admitted that if it were not for Isabella, she would have never gained 50 Accomplishment Patches in a single day. Interestingly enough, Candace doesn't try to bust Isabella when Baljeet and Buford pose as Phineas and Ferb, even though she acknowledged that Isabella was in the backyard. This may mean that they are very good friends. In a future episode, revealed during a talk session with some of the Phineas and Ferb cast and creators on July 9, 2011 in New York City City, Isabella will ask Candace for romantic advice. When she is sad or worried, including when her dog Pinky was missing, Candace acted like a big sister to her as she tried to cheer her up, letting her know that Pinky will turn up like Perry does.
As the troop's leader, the Fireside Girls always follow Isabella's orders, always ready, and willing to do whatever it takes to help their leader in helping Phineas and Ferb, even risking themselves to do it, like driving a van to get sap. Although the girls are loyal to Isabella and have earned many badges in helping the boys, they have voiced that they've collected more than fifty of the 'helping thy neighbor' badge because of it. They're also aware of Isabella's crush on Phineas and her desire to marry him, and they jokingly tease her for it. Even so, they remain friends with the boys and squealed with delight when Phineas invited Isabella on a romantic cruise. Also, Gretchen sometimes reminds her of what they were going to do when the troop needed to get sap from a rare tree so the troop can earn their "Sap Collecting Patches".
Isabella petted Perry in "Rollercoaster", saying that Phineas and Ferb could teach him some tricks. She also sang "Come Home Perry" with the boys, suggesting she likes him as well, even though she has Pinky as a pet. She petted Perry again in "Phineas and Ferb the Movie: Across the 2nd Dimension" while hitting a baseball off of him out of the backyard.
Isabella likes Pinky and feeds him grilled cheese, but he, like most other dogs will run after cats. Isabella also seems to have given him some orders to him as he almost ate Candace, but Isabella told him to stop, which Pinky did. She also took him to the Fireside Girls clubhouse one day, but he left since he was an O.W.C.A. agent.