"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAH" Izuku yelled at the top of his lungs, his voice mixed with a feline roar... just as the musclebound leopard in the mirror did the same.

Inko heard the scream from the kitchen, instantly getting worried. "Izuku, dear... Is something wrong?"

Izuku began to hyperventilate. With his hands he touched his face and moved his hair, seeing how the leopard in the mirror did the same... until he realized that he didn't have hands, but paws, with cushoned pawpads on his fingers and in the center of his palms. He tried to look at them closely, but as he tensed his fingers, long sharp feline claws popped out, nearly gouging out his eyes.

Finally Izuku turned around to appreciate the rest of his body. His sudden growth had caused his pajamas to shred, leaving him in only his All-Might brand boxers, and looking down at his legs he realized how tall he had become. At that moment, his feline nose picked up a somewhat foul scent. Lifting his muscular arm and sniffing his furry armpit, he realized that musky scent was coming from himself.

"Ugh! Yuck!" Izuku said, wrinkling his nose in shock and disgust.

Taking a couple steps back, he bumped with a wall, causing Izuku to notice a new appendage on his lower back: a long fluffy tail. Trying to see it, Izuku turned around several times, inadvertently knocking over several of the things in the bathroom.

Inko, concerned about all the ruckus, went to knock on the bathroom door. "Son! What's going on?"

"No, everything's fine in here!" Izuku answered to calm her mother down... without realizing that his deeper voice only managed to upset her even more.

"Wha... Who's there? You don't sound like my son!" Inko exclaimed indignantly.

"Seriously, nothing's wrong!" Izuku repeated, but as she reached for the door, he tensed his fingers again, causing his claws to dig into the door.

Freaked out by the sudden scare, Inko decided that it was too much, using her emergency key to open the bathroom door just as Izuku retracted his claws to wriggle out of the door.

"AAAH! An intruder! Help!" Inko screamed, seeing the huge cat person on the bathroom floor..

"No no no! You don't understand..." Izuku tried to explain, but Inko had already armed herself with a broomstick.

"Izuku! Izuku, wake up! Call the police! There's a thief in the house!" Inko yelled at her son, who she thought was still at his room, hitting the one she thought was a thief with the broomstick.

"Mom! Mom! It's me! It's Izuku! Stop it!" Izuku pleaded, trying to avoid the broomstick. Following his instinct, Izuku raised his fist and, using nothing than his index finger, he hit the center of the broom with such strenght that it immediately split in two.

"I...Izuku? Darling?" Inko said, looking at the stranger more carefully, noticing his green hair...

"Yes, mom... it's... it's me..." Izuku tried to explain, relieved that his mom was finally getting it.

"Izuku... But what happened to you?" Inko asked.

"I don't know..." Izuku commented, standing up and surpassing his mother's height by several centimeters. "I just... woke up and... and I looked like this! And the tail and the claws..."

"Could it be..." Inko pierced it together. "What if this is your quirk?"

Izuku froze for a few seconds. In his surprise at his amazing transformation, he hadn't considered the possibility that such a sudden change was the result of a mutation-type quirk. "Maybe...?" He looked at himself in the mirror. Now that he had accepted it, he had to admit that his new body looked way better, feeling much more energetic than before.

Furthermore, Izuku looked at the broken broomstick on the floor, split exactly by half... How... and why did this happen?

"Oh, Izuku..." Inko murmured, with tears of joy.

"I have a quirk... I... I have a quirk! I HAVE MY QUIRK! So... I can be a hero, right?" Izuku asked, excited.

"Yes, Izuku... now you can be a hero!" Inko said, hugging her son. "I'm sorry if I didn't believe in you before!" The hug lasted a few seconds, as Inko held his feline son tight.

"Oh but...mom? I think I had an... accident..." Izuku moved away from his mother a bit to show the torn pajamas caused by his sudden growth.

"Oh! Don't worry dear, I'll have to go and buy you some new clothes in your size. We'll also have to go to the doctor to check out your new changes, and... And you haven't had breakfast! Oh, Izuku... I'm so proud of you!" Inko mumbled excitedly, wiping away her tears and leaving her son in the bathroom.

"Sure mom, thank you very much!" Izuku replied. He again turned to the mirror, to see his new feline face more closely. He would never have guessed that his quirk would be mutation-based, as he imagined himself having a power similar to All-Might's. Still, he wasn't going to complain after all...

Flexing both arms, Izuku marveled at his powerful fur covered biceps. He was eager to test drive all his strength. "Hehe, great...I'm going to enjoy this."

~~Later~~

Inko called the school, telling them that Izuku couldn't go to classes that day due to a "medical emergency", and after borrowing some clothes from a neighbor (whose eldest son had a bull-based quirk), they went to the medical center where Izuku was checked out.

The doctor was impressed by the sudden appearance of a mutant quirk at his age, although since there had been similar cases in the past, it was plausible that he would have changed at any moment.

After that, Izuku and Inko were taken to a heteromorph quirk specialist. They did a few reflex tests, an x-ray, and an eye exam, to check for all the changes in his body due to his quirk. They played a documentary for them, explaining the various phisical characteristics of a snow leopard, their physiology, abilities, tips for fur maintenance, among others.

After that, they both went to the mall to buy suitable clothes for Izuku. Inko went to a specialty store to buy cat-related gadgets, such as a bell, a scratching post, a large fur brush with shampoo for specialized fur care, catnip, etc.

Meanwhile Izuku headed to various clothing stores to match his new style and size: in particular, sleeveless shirts that didn't impede mobility, loose but comfortable pants, and fingerless gloves that protected his paw pads. However, he decided to stay barefoot for the time being.

As he walked through the mall, he couldn't help but notice the girls around him whispering. Before, it was because he didn't have a quirk and they felt sorry for him, but now, with his highly developed hearing, Izuku could hear clearly that they considered him intimidating, cute, and in the case of a girl, even handsome. One girl even bumped into him, and as he helped her up, Izuku noticed her face turning red like a tomato.

"Heh, I wonder how they'll react when they see me at school," Izuku said quietly.

Very late at night, Izuku and his mother returned home, carrying all the things they had bought for him. Izuku was exhausted so he headed straight to bed. He felt as if everything that happened was nothing but a wonderful dream, but the tail wagging behind him ensured him that it wasn't.

"Tomorrow will be the best day of my life." Izuku thought as he drifted to sleep. That would be the beggining of his path to becoming Japan's no. 1 hero.