Fred slammed the door with a huff and started walking towards the pond. It was a rainy day, so no one was out there—which is what he preferred when he was frustrated. That was a difference between him and his twin.
When George got frustrated, he would do one of two things—either shut himself into a room (usually the bathroom where no one could bother him) or go out to a pub or a club where he could lose himself in a crowd. It would depend on how bad he was feeling—the worse off, the more he wanted to be around a bunch of strangers. Fred was the opposite. If he was feeling a little irritated, he wanted to be around people. As it got worse, he wanted to be alone with his thoughts.
By the time he got to the pond, he was soaked through and his short hair was flat to his head (long hair didn't go well with potion making—the potion exhaust made everything greasy. The twins agreed that shorter hair was better than looking like Snape). Fred bent down a picked up a decent sized rock. He tossed in in his hand a couple of times before chucking it into the middle of the pond, putting enough force behind it to make a pretty big splash.
Off to the side he heard a startled squeak. He turned and saw Hermione sitting under a tree with an umbrella, reading. Fred groaned. Just my luck.
"Fred! What are you doing? Trying to scare me to death?" Hermione shrieked.
"Yeah, sorry," he snapped, turning around to walk into the nearby forest—maybe it would be peaceful there.
Before he could, Hermione spoke up again; this time gently. "Wait, Fred. Are you alright?" she asked, putting her book away in her bag.
Fred clenched his fists, but it didn't help to keep himself in check. "No, I'm not alright. My parents think that I'm lazy and a waste who won't grow up and do something meaningful with my life, my brothers think I'm an idiot who doesn't know anything, and I want to be alone so I can demean myself until I feel better, but apparently that is never going to happen!" he ranted, finishing with a yell.
Hermione's eyebrows were as high as they could go. He could understand why—it was very rare that he got this frustrated so not many people (ie two people—George and their father) had seen this side of him. Fred sighed, actually feeling a little better after that outburst. "I'm sorry, Hermione. I just…exploded."
Hermione pushed the shock from her face and nodded in understanding. "That's alright. Sometimes you just have to let it out." She paused, as if trying to decide something. "Would…would you like to sit?" she asked awkwardly. Fred cocked an eyebrow. "You can keep talking—letting off steam," she said.
He thought he could hear an undertone of hope in her voice, so he nodded. "Sure. Just…don't tell me how stupid I apparently am, because I've had enough of that," he warned.
Hermione nodded and scooted over while he sat next to her, still holding the umbrella. It wasn't quite big enough to cover the both of them, so after he got settled, she scooted closer (to Fred's astonishment) until their sides were nearly squished together until they were both under the umbrella.
They were silent for a while, feeling the tension in the air, until Hermione broke the silence. "I don't think you're stupid," she said quietly.
"What?" Fred turned his head to look at her in surprise, while mentally marveling at her tanned skin, dark hair, and chocolate eyes. How had he never seen how…exotic she was before?
She sighed and faced him. "I don't think you—or George—are stupid. In fact, you are rather brilliant. The magic that you can use is just incredible. I do wish you wouldn't use them for pranks, or for more than just pranks, but that doesn't change how advanced it is."
Fred's mind felt blank. He couldn't believe what she was saying. "Wha…wait… Really?"
"Well, yes. Sometimes, when you are talking about a new candy or object, I want to corner you and George and beg you to tell me how you did it. What spells you used or how exactly you were able to coat the candies with the potions for the desired effect."
Fred couldn't help himself—he started to grin. "Well, get comfortable. Because you are in for a lecture on the beautiful art of prank-making," he said, wrapping an arm around her shoulders and tugging her back so she was half-leaning on his chest.
He started talking about the shield hats, knowing that those would grab her attention and maybe get her more comfortable. When he got to the part on how he and George would enchant the hats, she became very interested. "Wait, you enchanted the hats to hold the shield charm so it wouldn't fade? How? Where did you learn to do that?"
Fred shrugged. "When Harry found the Room of Requirement, we went there once in a while to see if we could find an answer to some of the problems we were having. That was one of them. The Room had a whole diagram of how to enchant an object. Pretty simple once you get the hang of it."
Hermione's eyes widened. "That…is amazing!"
Fred grinned smugly. "It is, isn't it?" She smacked her hand across his chest, but she was chuckling. "I'll teach you later, but we are still on the theory part of this lesson. So sit back and relax."
Hermione smiled as she settled back into his arm. Fred couldn't help enjoying that she was leaning deeper onto him. He was a guy after all.
"So, knowing that we have a fair understanding of enchanting, you can understand how a lot of our stuff works," he said, continuing talking about different objects. He told her all about their joke items. How they could make some of their fireworks move as if alive (like the dragon they set on Umbridge). The decoy detonators activated upon hitting the ground, no matter what height it was dropped from. He told her about many of the products at the shop—and even some that they were working on.
However, he didn't tell her how the Patented Daydream Charms worked. "Oh come on. Please?! It is extraordinary magic. Please tell me?" she begged.
"Nope, the only way for you to learn that would be if you either worked for us or married one of us," he said in a teasing manner, though he could feel the tips of his ears warming. At least she was still tucked under his arm so she couldn't see the turning pink.
She huffed. "I will get you to tell me someday, Frederick Weasley. That I promise you."
"I look forward to you trying."
She sighed, releasing the tension in her shoulders. "So…what's next?"
"Well, you wanted to know about the potions we used, right?"
"Yes. I was curious about your candies. How did you get them to cause the right effect? From what I've learned, coating candies is unreliable."
Fred shrugged gently to avoid jostling her. "We didn't coat candies," he said simply.
Hermione lifted her head a little in order to look up at him. "What?"
"We didn't coat any candies," he repeated. "We just made the potions into candies."
Hermione blinked in shock, absorbing his words. After a moment or two she exploded into action—sitting up (causing the umbrella to fall to the side) and turning around to look him in the eye. "Are you serious?! You made the potions into candies?! How?! How is that possible?! Potions are liquid and there had never been any ingredient found that could change that."
"We found one," Fred interjected.
"WHAT?! What is it?" she shrieked.
Fred backed away from her a little bit. "Gelatin…?" he answered, almost afraid of her reaction.
Hermione's mouth dropped open. She rubbed her face with her hands. "I can't believe it," she murmured. "I can't believe that no one has tried that before."
Fred stayed silent—watching Hermione go from shouting to nearly whispering was rather unsettling and he didn't know what to do. He even flinched a bit when Hermione turned to speak directly at him again.
"Do you know what kind of a break through this could be?" she asked. "This is incredible—it could change the way the world does potions!"
Fred turned his head, shying away from her. "I don't…know… Really it's nothing special."
Hermione reached out and placed her hands on his cheeks, forcing him to look at her. "It is, Fred. It really is special." He went to turn away again, but she held fast. "I know everyone seems to think you and George aren't intelligent, but you are. You are more than intelligent—you are absolute geniuses! And I think that anyone who can't see that is the idiot."
Fred found himself drowning in her eyes and in her commitment to his intelligence of all things. He couldn't stop himself—her hands were on his cheeks and she was just so close. He reached out and pulled her closer, one hand on her back, the other tangled in her hair, catching her lips with his.
She was still for maybe a second before she sunk into the kiss with a silent sigh, moving her hands from his cheeks to his shoulders. He tugged her even closer and before either knew it, she was straddling him lap.
Hermione ran her hands over his shoulders, down his chest, then back up to his short hair—just barely long enough for her to grasp. Fred's own hands traveled as well, from her hair down her back, and settling on her bum. Her mouth opened in a gasp when he squeezed and he snuck his tongue inside, massaging hers. She was timid at first, but a fast learner and soon he found himself in a battle for dominance.
He could feel himself starting to get aroused and he decided they needed to cool things down before something happened that she would regret. He most likely wouldn't have regretted anything. Although he was sure he was going to be really frustrated now.
He withdrew his tongue and started to slow their snogging. Instead of following his lead, she pushed against him harder and bit his bottom lip. He groaned and rolled them so that they were lying on the wet grass next to the tree with him on top. His hands roamed against her sides, trailing over her breasts down to her hips and back again.
Hermione's hands tangled against his neck, playing with the soft hairs there. Fred released her lips and began nibbling and licking her jaw line down her neck where the sensitivity there made her gasp, then up again to her ear.
"We should stop soon," he murmured into her ear, licking up the outer cuff before continuing, "or I won't be able to stop at all."
"Alright," she whispered breathily, feeling the raindrops on her face. She grabbed his face and brought it back to hers for another searing kiss. "I don't want to, though," she murmured once she released him.
He leaned his forehead against hers. "Neither do I."
They laid there for a while before Fred rolled off of her and helped her sit up again. The ended up in the same position they were when Fred had begun telling her about his inventions, though without the umbrella. He rested his cheek against her wet hair while she had her eyes closed, listening to his racing heartbeat.
"Hey Hermione," he said against her hair.
"What are you doing tomorrow evening?"
"Nothing. Why do you ask?"
"Would you like to go to dinner with me?" he asked.
She smiled and he could feel her nodding against his chest. "I would love that."
I don't have any particular idea of when this takes place. It could be the summer before HBP took place, or it could be after the war (in the Fred-didn't-die universe). You can choose whichever you prefer.