This fic is not related to my previous Phineas and Ferb fanfic, How Ironic!

At the height of their 20th Anniversary, Isabella's parents decided that the world is too big to stay in one place. Hence, they packed their bags and left Isabella with the Flynn-Fletcher's. It was the start of their high school year and, while Isabella is glad that her parents are finally off her back, it is a bit weird that she's to spend her whole high school life living with her childhood crush—who she's totally over of. She is definitely over Phineas. It didn't take long for her to get settled in. She practically spent her whole childhood on their backyard and Mrs. Flynn-Fletcher would often invite her over for dinner. It only seemed like she was moving into her second home.

Isabella plops herself on her single bed, feeling glad that Candace lent her her room when she went away for college. It has been three years since she's been living in their home and the oldest Flynn is currently studying journalism in the hopes of becoming a gossip writer for one of those famous women's magazine. Isabella might deny this if asked, but she kinda misses the erratic female company.

"Bella, supper's ready."

She lifts her head at the sudden voice from the door, unaware of it even opening. Standing at the doorway is Ferb wearing his sweatpants and nothing to cover his upper body. Despite her awkwardness with Phineas' un-evolving obliviousness with her previous feelings for him, Ferb is the other reason why she still feels a bit uneasy after all these years with her living with them.

It all started freshman year. Ferb, the boy who would always always be by Phineas' side and nowhere else, came home late. Mr. And Mrs. Flynn-Fletcher, not used to him arriving later than Phineas, started panicking. As usual, Isabella's fireside instincts kicked in and she managed to calm them both with a cup of tea. A few hours later, Ferb came in wearing a hoodie—a goddamn hoodie—and sweatpants with a towel around his neck. Candace almost tackled him with need for answers and he merely shrugged and answered in that cute British accent of his that he's been to the gym. It took some getting used to but it eventually became a routine. They would all wait for Ferb to come home from the gym and eat dinner together—of course, after he showers. But what troubled Isabella the most is that he seemed to appoint himself to be the one who always calls for her whenever dinner was ready. Not that she's complaining about his chivalry but, quickly after he's been exercising at the gym regularly, he developed these abdominal muscles that definitely spend too much time on her imagination. Even the girls at school began to notice the physical change in Ferb. They all called her lucky that she lived with the British stud and his charming brother. Pssh. Agonizing is the word that she would use living with them.

"Bella?"

She realizes that she's been ogling his bare, sculpted chest for quite some time. "You know you could just use the nickname that everyone uses for me: Izzy,"

The smirk on his face is enough to send butterflies swarming her stomach. "Then I wouldn't be special."

She stands up from her bed and sashays towards him. "Who says you're special?" With a quick kiss on his cheek, she leaves him grinning to himself.

It's not just hard for Isabella. It's also hard for the brothers too. Adorable or not, Phineas and Ferb are hormonally-infested teenagers.

For Phineas, it's like suddenly realizing how attractive his best friend is. When he was eight years old, he's always found her helpful. The way she's always there to help has always cheered him up. When middle school came, he had to endure comments from his guy friends on how cute Isabella is. It continued till high school where comments turned from 'cute' to 'Isabella is goddamn hot! Hey, let's make a bet on who gets to make out with her first' or 'Damn Flynn, you're so lucky to be living with a hot ass like that'. Of course, his guy friends spent most of their free time in the Flynn-Fletcher home in the hopes of landing a date with the sweetheart.

It's not like he likes her. He's just mildly attracted. Isabella has always been cute but watching her turn into a bombshell at every party they attend is hard for a teenager like him. He's never really dated much and only spent four months at the most with a girl before he becomes bored because...well, teenage girls only like to talk about themselves and their social status. Teenage girls that do not include Isabella. It seems like his childhood friend has not only grown beautiful but also intellectual. He found himself always in a verbal battle with her during dinner while his parents and Ferb watched amusedly.

"She liked you when we were kids, you know?" Ferb told him this one night at their usual midnight musings. Even if they're in high school, they do not attempt to change rooms since they've been always comfortable with each other's presence.

This startled him awake because this is new information.

Ferb snorted in the bed beside his, also sitting up. "Tell me you haven't been that oblivious, brother." Ever since his stepbrother started going to the gym, Phineas has always been conscious of how his brother is snagging all the attention with no trouble at all.

"I had no idea!" Oh how things could have changed if he knew.

"It doesn't matter now. She's over it." And that's when he knew how late he really is. The sharp edge in Ferb's voice only proved that his brother has some feelings for their current housemate.

The next couple of days, Phineas noticed how the two of them would keep sending each other subtle glances. Before, this was only a shrugging moment for him that Ferb and Isabella are only curious about each other. Now, he couldn't help but also notice the way the two of them would gravitate towards each other during breakfast...the way his brother would escort Isabella towards her locker everyday at school and the latter smiling like Ferb just saved the day whenever he offers to carry her books for her...how both of them would be smiling so wide everytime both of them comes down for dinner...how Isabella sends a wink towards Ferb every night when she says good night to them wearing those short nighties. Yes, he's too late. Definitely too late.

Unlike his American brother, Ferb has always known about Isabella's beauty. He knew ever since he first met her when she and her mom welcomed them in the neighbourhood. She was wearing a puffy pink dress, hiding behind her mama's skirt. Ferb, more talkative then than he is now, smiled widely at her and asked her if she wanted to play. Of course, their friendship was put aside once the young Isabella set sights on the more cheerful and friendly Phineas. Ever since they were kids, Phineas always gets more friends. It seemed like the cute British boy is bound to be the cute loner.

He had to endure her fawning over his stepbrother their whole childhood. He could not help but think of how unfair everything seemed. It was not fair how Phineas got the friends and the girl. Still, he could not let the envy invade him and he settled in comfortably as Phineas' assistant. He even managed to let go of his little crush with Isabella and exchanged her with the darker and older Vanessa Doofenshmirtz.

This all changed in middle school after their small, yet highly functional, family returned from their vacation at the Caribbean islands. Puberty kicked in for the stepbrothers yet it seemed that it beat the hell out of Ferb. He didn't bother cutting his spiky hair during his short bohemian phase during their vacation. Lawrence Fletcher told his son that his developing green curls were inherited from his birthmother. When they returned to Danville, his friends were all surprised that his spiky hair was gone and was replaced by soft, luscious curls just resting two inches above his neck. Not only that but his voice developed into a husky British accent that won over every girl as soon as the first word from his mouth comes out. Suddenly, he started getting all the girls' attention. His athletic abilities also stood out and every guy wanted to be his best friend. His attitude didn't change though. He's always been comfortable behind Phineas' shadow.

It took him by surprise when his parents announced that Isabella would be living with them temporarily for the start of their secondary education. Their friendship endured the confusing minds of tweens so it would not be awkward for him to be seeing so much of her. After all, she—besides Phineas—is the only other person in their school that he's comfortable talking with.

Comfortable is definitely not the word. Isabella is a nightmare to be living with. He discovered this a few months after she settled in. She has made it her mission to make him more talkative by constantly teasing him. There is one time that they swore never to talk about again. Ferb acted on his hormonal teenage male instincts and took a willing cheerleader home while his parents spent another night in an antique convention. Isabella's daily student council meetings ended too early and she caught him in a very compromising position. They both blushed furiously and, for weeks, ignored each other as best as they could.

"Hey, can we forget all about what happe—oh my God!" Isabella burst inside his room one day, finally fed up with their silent friendship. Her hands covered her eyes as soon as she discovered him doing push ups wearing just his sweats.

His next action changed the course of their friendship. Feeling a bit mischievous, Ferb stalked towards her like a prey and wrapped his muscular arms around her lean waist, pulling her inside his room. His other arm closed the door behind her and he found himself caging her small body using his muscular one. He blames it on her tank top and boy shorts.

"It's okay to look, love. I'm not in the nude." He pried her hand away from her face.

Her sharp intake of breath pleased him. "What are you doing, Ferb?"

He merely shrugged, bending over to tease her a little bit. When she closed her eyes in anticipation, he stopped leaning in. "You were telling me something?" He whispered once his lips are close enough to brush against hers.

This embarrasses her as her eyes fly open and her fingers stopped clutching his soft green curls. "I—I was j—just...fuck...are we okay? I mean, c—can we just forget what happened a week ago?"

He decided that he liked it when she rambled so he teased her a little bit more. "Oh, you mean your ogling at my chest?"

A snort escaped her lips. Her fire is back. "I did not ogle at your chest! I was referring to me witnessing your hormonally infested activities with cheerleader Mindy!"

"Oh...that."

"Yes, that!" She pushed him away with her tiny hands. "So are we good?"

He shrugged, basking in the glory of witnessing the darkened blue of her eyes. "Yes, we're fine."

"Good," She says with finality before throwing his door open and rushing out.

"You look divine when you're angry!" He shouts after her, laughing loudly when he hears her cry of frustration.

He definitely likes frustrated Isabella.

Ferb regrets his actions. Their short teasing episode only resulted in turning the game on for Isabella. First, it was just intriguing slow licks at her spoon during breakfast while she stared at him tauntingly. He couldn't count the amount of he had the urge to take a cold shower in the morning because of that. Then, it turned to her wearing scraps of cloth that she waved off as her night clothes because of the heat of the impending summer. Her 'clothes' showed too much skin that even sweet, oblivious Phineas has drool running down his mouth every time she bids them good night.

What broke his crumbling control is coming home one night when he thought that he had the house to himself for the first time since Isabella moved in. Mum and Dad joined a couples camping trip with Baljeet's parents. Phineas was spending the night with his science club friends after going on a date with Ginger. Isabella announced this morning that she will catch up all night with her previous Fireside girlfriends—no doubt only agreeing to stay over because she wants to grill Ginger about her date with Phineas. It pissed him off, to be honest, that she might still harbour some sort of crush for his stepbrother.

He took a refreshing shower, washing away all the sweat from the gym. He didn't bother taking any clothes with him since he's alone. Hence, that is how Isabella found him. As soon as he saw her inside his and Phineas' room he realized that he has nothing but a towel wrapped around his waist just below his v-lines.

"I thought you were sleeping over with the Fireside girls?" He inquired, raising his dark eyebrows at her sitting form.

She nodded, a small sly look on her face. "I was but then I decided that there is no place like home."

"Okay?" He ran his hand through his wet locks, taking large strides to his underwear cabinet.

"Ferb?"

"Yes?" Where are my lucky green boxers?

"It's laundry day yesterday and I hope you don't mind..." Isabella trails off.

He turned around and the liquid on his throat dries up instantly. Holy fuck. She was standing in front of him, wearing his green boxers and loose fitting white shirt. It looked like she's wearing nothing else but the shirt because of its length. The green just resting above her thighs proved otherwise. Her dark hair was tousled like she messed it up while he was looking for his boxers. She was not wearing any makeup but the obvious smudge under her eyes suggested that she previously washed it off.

"Holy cow," She looked like she just spent the whole night making love.

Her hips swayed agonizingly as she walked towards him. "Is this alright, Ferb?"

"Fuck it." He all but pushed her on the nearest bed, taking her mouth in his. She tasted of sweet strawberries and mint and he couldn't get enough of her. His hands found themselves under the confines of her—his—shirt. Holy smokes, she's not wearing a bra. She moaned as he trailed his fingers down the edges of her stomach, wrapping her long legs on his waist. The skin to skin contact is enough for him to groan in anticipation. Is she really doing this with him?

Her hands were everywhere, either gripping his wet hair or trailing her nails at his bare back. Then at his chest...pushing him flat on his back with her straddling him. His tongue traces her bottom lip, a groan escaping him when she allowed him access. He has absolutely no self-control in the moment.

Then she's gone.

"Good night, Ferb."

Phineas is going to murder him for what they did to his bed. "Bloody. Fuck. Me."

Aha! Another idea that I simply cannot get out of my mind. Hope you like it It's a bit AU since i didn't have Phineas and Ferb build any contraptions.