A Misunderstood Deku

Beginnings

Here goes nothing... She opened a blood bag, and when the scent reached Izuku, it shook him like a thunderbolt. All his focus was on it, and nothing else. He resisted against the rope, as much as he could, trying to get to it, but it was clear he was also trying to stop himself mentally from going berserk.

Inko, proud to see Izuku maintaining control, continued to offer words of encouragement. "You can do it Izuku! I believe in you! You want to be a hero like Allmight, right? This is a big step towards that!"

After hearing that, he pulled himself back toward the chair, fighting against his instincts, as if having that goal made it easier to resist. Yes! You can do it!
She watched as he held back, waiting 5 minutes... until she then closed it, sealing off the iron tinged aroma. Placing the bag on the counter, she then kneeled to him, embracing the green haired boy in a hug. He sank forward into her embrace, exhausted.
As he drifted to sleep, she congratulated him in a hushed voice. "Very good! We'll keep doing this every night! You'll master this in no time!"

The next day, Katsuki Bakugo waited by his locker, waiting for his best friend to arrive. When he did, he was surprised by his more raggedy appearance. "You look... worse..."

Izuku rubbed his left eye, getting sleep out of there, before telling the blond 4 year old boy: "My quirk needed training..."

That got Katsuki to immediately shift to a happy expression of wonder, "You know what it is?!"

He nodded, smiling widely. The blond noticed his larger canines as a sign of it, but nothing else. Clearly, it was related to blood, which was cool in his book. "Yep!"

He leaned over, eager for details. "Can you tell me?"

Unfortunately, Izuku shook his head. "Mommy said to tell no-one yet..."

That confused him. Why would Auntie want him to hide his quirk? Isn't having one amazing? "Ok?"

They went through their day like normal. For a few months, the new normal was established. School, homework, early evening bloodlust training, then dinner and sleep. As the years went by, his skin became paler, and his hair darkened, and he learned to control his urges. Nobody really noticed except Bakugo, who knew that had to be a part of his quirk. He'd had suspicions on what it could be, but nothing definite. The blond wouldn't push it, though, as he trusted Aunty Inko's judgment. However, one day when Izuku arrived back home, he found the house to be too quiet. "Mom?"

He could smell the tang of iron emanating from the kitchen. Fortunately, his training paid off, but after a moment, Izuku realized that it wasn't from his mother's cooking. Nagging worry turned to horror as he bolted into the room the aroma came from, now worried for another reason. "Mom!"

Running in, he found her resting against cabinets, bleeding profusely. She was still alive. The greenette didn't know how long she was there, but the woman turned to look at him. Fear confined to strike as she asked weakly, "Izuku... call for help..."

"I... I will!" He quickly grabbed the house landline and called for an ambulance.

Fortunately, an ambulance arrived quickly, and the EMTs put his mother on a stretcher. However, before she was carried away, she gave Izuku a note. It was the doc's note, about his diet. From all those years ago. "Show them... if they ask... Keep it with you."

Tears started to well up as he denied what was clearly happening. "You'll be fine! You have to be..." All of the Emergency services personnel were accommodating, considering the circumstances. A female police officer and one with the head of a cat stayed with him for a few hours, providing reassurances... but they both knew that was a terrible set of injuries.