Harry knew he wanted to marry into the Weasley family when he received a hand-knit sweater from the Matriarch. He'd asked Ron and the twins about his siblings and even been shown pictures of them.
When not long later in the year they needed to send Norbert off with Ron's older brother, who apparently works at a dragon reserve, Harry decides he's the one.
Following the dragon Harry sent a note to the second eldest brother to declare his intentions of courting him, and that the Norwegian Ridgeback had been the gift sent along with it.
Charlie had laughed at the declaration of intent, he'd shown it to his boss, who'd also laughed at the idea of gifting a dragon as a courting gift, and sent a letter back telling the young Potter he was much too young to be trying to court him.
The second year Harry had sent holiday well-wishes, and christmas gifts as expected of friends, but at the end of the year he sent along a letter detailing his rescue of the youngest Weasley and his battle with the Basilisk, as well as a courting gift of one of the massive venom fangs and a handful of smaller rear fangs from the Basilisk, carefully wrapped to avoid accidents.
Charlie had been impressed at the gift this year, and after explaining the situation to his older brother Bill, got some help from the Goblins in crafting the fang into a dagger, and the rear fangs into jewelry. His boss laughed at him for entertaining the young heir, even when he wasn't around to see the progress he was making. Charlie had written back thanking him for his actions, he didn't mention the courting but had included a picture his boss took of Charlie wearing the jewelry and with the dagger in a sheathe on his thigh, he was digging through a dragon nest counting eggshells with a dicta-quill floating next to him writing whatever he was saying.
For the third year Harry followed his own example and sent only holiday well-wishes and christmas gifts, he doesn't tell Charlie it was the picture of him wearing the Basilisk jewelry that allows him to cast his patronus. At the end of the year he sends a torn piece of Dementor cloak from one he'd banished, as well as a parselscript book about chinese serpent dragons with a hand translated copy he found in the Chamber of Secrets, finished with a picture of him standing beside his patronus, which took the form of a lion, with a small "Thinking of you." Written on the back in parselscript.
Charlie is excited as he shows off the books to any of his coworkers that would stop and listen, several doing so in their own excitement to know what secrets the book holds. His boss helps him translate the parselscript on the picture and seems way too excited about Charlie's courting. He sends back an excited letter talking about the dragons mentioned in the book, a terribly translated but still decent attempt at parselscript for someone who isn't a parselwix, and a picture of him wearing the dementor cloth like a bandana his boss took while he was cleaning a room that'd held a sick dragon.
The fourth year is when they meet for the first time. Charlie, blushing bright red while wearing his courting gifts, nearly matched skin to hair color when Harry bowed and kissed his hand before turning to the rest of the family and announcing his intention to marry Charlie. Charlie's older family were excited and congratulating him, Ron and Ginny were explaining how pureblood courtship worked to Hermione.
Harry finds Charlie after the first task and asks him to stay to attend the Yule Ball with him, as he has to have a date as a champion but doesn't want to take anyone else. Charlie's boss had already given him the time off when he asks for it later, with a smug but entertained grin.
Harry sent Charlie a beautiful sapphire blue set of dress robes, and a gold lion cloak pin to match his basilisk fang jewelry, clearly hoping he'd wear them. When he showed up Harry was wearing emerald green dress robes fitted to him, and the same gold lion cloak pin, as well as his own basilisk fang blade in a holster attached to his thigh. Harry was terrible at dancing, he'd warned Charlie in a whisper minutes before entering, making Charlie let out a hearty chuckle, Charlie took the lead while dancing, and they danced for far longer then was expected of them. Harry escorted him back to the gates where he could apparate, and Charlie gifted him with a soft and fleeting kiss followed by a cheeky grin at the dazed look on the youngers face.
At the end of the year Charlie isn't expecting a gift after everything that happened at the end of the tournament, he tries to get time to visit but the Order tells him to stay in Romania. Harry sends him the little Hungarian Horntail statue, the golden egg he'd pulled from her nest, and a copy of the marauders map.
Charlie's boss was incredibly jealous of the tiny dragon that'd now claimed the golden egg as it's nest. When opening the egg in an experiment they found Harry had replaced the mermish with his voice saying something in parseltongue, but they couldn't translate it. Charlie nearly cried when he'd activated the map and was able to watch his younger siblings move around the school and know for a fact they were safe. He sent back a very heartfelt thank you and a note that they couldn't translate the parseltongue, as well as a picture of the little dragon sleeping on the egg.
For the fifth year they exchanged regular letters every week, discussing everthing from favorite colors to what they wanted in life. Harry was sure to inform him of how terrible Umbridge was, and how him and a bunch of his classmates didn't think they'd pass the year. Charlie didn't ask why they disappeared off the map regularly, assuming Umbridge was reading the students mail. Halfway through the year Sirius Black gave Charlie his blessings when he visited the burrow for christmas.
At the end of the year Charlie again wasn't expecting anything, with Sirius's death still heavy on his mind. Harry sent him a copy of a letter Sirius wrote approving of Charlie and Harry's match after he'd met him along with ablack family promise ring, a book the marauders wrote about becoming animagus, and a note about how he'd given his tournament winnings to the twins for their joke shop.
Charlie's boss had promoted Charlie to her position this year, but he did send her a picture of the promise ring on his hand (that he'd copied to send to Harry) along with the letter about how the reserve was holding uo without her.
Sixth year sees little changes, instead of talking about Umbridge now Harry takks about meeting Dumbledore, and how terrible Snape is at teaching Occlumency, and his half blood prince book.
After Dumbledore dies Charlie just recives the Half Blood Prince book and a picture Harry had clearly taken the night Hagrid came to see the dragons, Charlie was the subject standing with his arms outstretched in front of the angry Hungarian Horntail.
Charlie sent back a picture of the newly hatched Horntails.
Charlie hadn't been expecting Harry at Bill's wedding, so he'd gone alone. He was Bill's best man, and Fleur was very happy to meet him, talking about how Harry wouldn't stop talking about him to her and Viktor. Viktor had clapped him on the shoulder and called him a brother. It was an odd experience for Charlie. What was even odder was a random Weasley he didn't recognize pulling him into an empty room and asking to dance. Charlie had denied him with an annoyed "I'm already seeing someone."
The random Weasley looking person got a really confused and hurt expression before looking down at his hands and having realization dawn on him. He moved his outer cloak to show Charlie the dagger on his thigh, and Charlie sees the lion cloak pin and nearly lurches forward to grab Harry's hands and apologize. They'd laughed it off and danced for nearly an hour in that empty room before it'd all gone bad.
The seventh year Charlie watched the map almost obsessively, watching his sister as she leads a rebellion against Death Eaters while he sits at home on the orders of a dead man, unable to act. He sobs when a dreadful looking dragon with broken chains flies into the sanctaury carrying a note from Harry, having not been sure he was still alive at all until then, asking Charlie to marry him when this is all over.
Charlie can't send a response, but he writes one anyway.
When the battle of Hogwarts is over, Charlie is standing with his family next to Percy's body, but he's watching the crowd for Harry, still in shock from seeing him dead, needing to see him alive to accept it's true.
Harry shows up after the funerals are over, knocking on the Burrow door at five in the morning with a consort ring in hand and a request for Charlie's hand in marriage halfway off his tongue before Charlie has already grabbed him and pulled him in for a tight hug.