Ernie/Hermione.
She found the first love note in her Potions book, two months after her break up with Ron. It was sweetly written, and spoke of longing from afar and admiring her courage and smarts.
The second note was found in her Charms book one week later, and was equally as delightful as the first. It spoke of desire to know her, to talk with her more on the topics she had brought up in class. It delicately touched upon the writer's wish to make her feel special and treasured.
The kind and loving words spread a warmth through her that she hadn't felt since the beginning of her relationship with Ron, and her curiosity about the writer grew.
The third note was delivered in person, and she was shocked to discover the giver. Ernie Macmillan stood earnestly before her, hope in his eyes as he passed her the folded parchment. She gently took it, and glanced curiously up into his eyes before carefully prising off the wax seal.
Will you go on a date with me?
The neat script she had become familiar with over the last two weeks peered up at her. Of all the people she had expected, Ernie Macmillan had not been one of them.
She wasn't displeased, she realised. Ernie was polite, and sweet and didn't appear to share the same disgusting habits that put her off Ron so much.
She accepted, and their first date went splendidly. As did their second, and their third, and they stopped counting and started spending more and more time together. They were both intelligent and hardworking, and neither minded the other spending extra time in the library.
Weeks turned into months, and while they stayed together she soon found that their relationship had become stagnant. They enjoyed each other's company, and cared for each other greatly, but the initial spark which had made their relationship interesting faded away.
Hermione gently explained this to him, and they broke up on good terms. Still, she went to Harry crying when it was all over, bemoaning her inability to find someone she could stay with.