I do not own Harry Potter, the Wizarding World, or any canon characters.
Note: This was the original prompt response.
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"Min, any luck?"
Minerva McGonagall sighed and looked at Poppy Pomfrey. "Sadly not. The Weasley Twins swore that they have played no pranks involving trip spells, grease spells, jelly-legs, clumsiness, or any other kind of spells that make a person trip."
"Drat, that was my best guess for the cause," Pomfrey muttered.
"Any luck finding a commonality between the injuries?"
"Aside from mostly being girls and the fact they are rather light injuries: black eyes, bruises, twisted knees and ankles, nothing at all. It's not even girls from a single House. It's all of them. Oh, they all have faces with differing shades of red and pink, but again, that's it. Aside from the fact that none wish to explain why they got the injury."
"We can rule out some kind of duel club, thankfully. I wonder what's going on?" McGonagall wondered.
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Like most things, it began with a seemingly innocent comment delivered teasingly.
Harry Potter was told by his best friend, Hermione Granger, that he was developing a bit of a stomach. It was getting so bad, one could see it despite his very baggy clothes underneath his school robes.
Harry looked down at himself and at the plate of treacle tart. With an expression no one had seen him make before, he looked about. Then he wrapped the tart in a napkin.
"Harry, what are you doing?" Hermione asked.
"Saving it for later. I don't want to waste it but I don't feel like eating it anymore right now."
She felt a pang. "I'm sorry, I was only teasing you."
He shook his head. "It's okay, thank you for telling me. I wouldn't want to get fat and fill out this shirt." Without another word he left, leaving a flummoxed Hermione behind him.
He wandered about the school, lost in thought. Being compared to Dudley, his brute of a cousin, even jokingly made him feel sick and anxious. He never wanted to be like the boy: wide as a tire with a wider cruel streak. He wanted nothing to do with that. He thought over and over as he paced, "I need a place to get fit, to get healthy. I don't want to be fat."
He stopped and looked at the wall, a door suddenly appeared. Normally that would be odd but it was Hogwarts after all. With a shrug, he opened the door. The interior astonished him. Gleaming gym equipment was arrayed in neat rows. Weights were organized along the wall. A bookshelf held all sorts of books like: Losing Weight is Magic, Wingainium Leviosa, Muscle Might is not Always Right but it Certainly Helps, and Getting Fit the Wizarding Way.
His eyes sparkled and he started reading and looking at the equipment.
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"You're here early."
Harry looked up, drenched in sweat. He smiled sheepishly at Angelina Johnson, Alicia Spinnet, and Katie Bell, his Quidditch teammates. "Yeah, I've been gaining weight recently and wanted to get a little healthier."
"For you, gaining weight is a good thing," Alicia said archly. "You're rather scrawny as it is."
He shrugged, "I guess but it was all around my middle and well..." he looked dreadfully embarrassed.
"Hey it's okay," Katie said soothingly. "Nothing wrong with putting on weight but doing it right. It's actually good you're doing all these workouts, we've noticed. You're putting Wood and the Twins to shame."
He smiled and the girls were struck by the open honest gesture. "Really?"
Angelina smiled back. "Really! Come on Harry, we'll help you get buff!
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The first incident happened a few weeks later. Harry had walked up to Hermione in the great hall. "I've been working out and my belly is a lot less pudgy now."
Hermione rolled her eyes. "Or maybe you let out your shirt some more," she said with a smirk.
Harry did what the Chasers told him to do if she responded that way. He lifted his shirt showing off his newly flattened abdomen showing defined muscles. "Nope! See?"
Lavender Brown was watching while eating and inhaled swiftly, which sent her sandwich down the wrong pipe. She coughed and started choking.
Harry immediately ran to her, put both arms around her waist and made a fist, pulling up. He was forced to learn the Heimlich Maneuver in the very real chance that Dudley choked. He pulled again and cleared the culinary obstruction from Lavender's throat. "Are you okay?" he asked worriedly.
Lavender's face was deep crimson for a myriad of reasons: choking on her food, coughing violently, seeing a surprisingly attractive Harry, and now being very conscious of his oddly toned arms wrapped around her midsection and him being pressed into her back. She managed to nod and squeak, unable to form words.
Harry thought he knew why she looked so uncomfortable and let her go. "I'm really sorry," he apologized hastily.
"No no," Lavender said hurriedly. "I didn't mind, I mean, it felt nice. I mean, th-thank you."
"You're welcome." Harry rubbed his neck and looked up, missing all the red faces on the girls around. "Anyways," he tried to say smoothly to a blushing Hermione," see? A lot less pudge."
"Yes, I see, saw. Very good, for your health," she stammered. "Harry, you know I was teasing you right?"
"But you weren't wrong either," he replied. "Now that I'm working out more, it's actually kinda fun so I really have to thank you." He patted her hand before taking his leave. He missed how scarlet Hermione went, and missed all the jealous looks girls threw at her, or the wistful ones that followed him.
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"You gotta act more positive to feel more positive," Katie said to him a few weeks later. "You know, it's okay to strut your stuff."
"I don't want to be Malfoy though," Harry said worryingly.
Katie laughed. "Unlike you, he has no reason to strut. You have plenty. Show off your hard work. Come on, like this. Back straight, shoulders square, chest out."
Harry did as she told him. "Huh, I kinda like how this feels."
Many liked how he looked.
Katie was one of them and she tried to will her flushed cheeks away. "That's great! Body positivity Harry! Now flex like Angelina does."
Harry grinned and did what the older girl does a lot, bringing his right arm up and flexing his arm muscles. If he were wearing his normal cast off clothes, the effect would have been minimized. Wearing the practice Quidditch jersey that the Chasers had surreptitiously resized to fit him perfectly emphasized the act.
Daphne Greengrass walked straight into a door, bouncing back and clutching her smarting nose. Luckily her companion, Tracey Davis, had missed the gaff, having walked into the wall beside the door also distracted.
Susan Bones' head had turned back so fast she gave herself a crick, wincing as she massaged her neck. The Carrow Twins had dropped their books, identically wide eyes on identical faces. Cho Chang and Marietta Edgecombe had stopped talking, staring blankly.
Katie could not keep the blush away this time. "Perfectly done," she praised.
He smiled broadly and tentatively hugged her, something the Chasers had been helping him get used to. "Thank you so much Katie!" he gushed. "I couldn't have done it without you!"
If looks could Curse, Katie would have been hit by more than a few at that moment. And it would have been worth it.