Disclaimer : Supernatural and Buffy are not mine.¤
The Winchester brothers were so busy with their own problems that they forgot about the strange girl they had met at that university. They had finally found their father and had just buried him when they heard about her again. It was at the Roadhouse, after the killer-clown case. They were drinking when a hunter at the bar interrupted one of Jo and Ellen’s argument, telling them that he had met a girl, probably younger than the Jo, hunting. As he was describing the girl in question, Dean finally asked, "Did her name was Dawn?"
The hunter looked at him, surprised. "Yeah. You know her?"
"Yep. Met her a while ago. She was at the university back then."
"Well, she’s not anymore."
According to him, she had been on the road for a few months already and she had guts. He taught her a thing or two, hoping to help her survive a little longer. The conclusion was that young girls could be good hunters. Ellen was not happy with the man and refused to serve him anymore. She was almost telling him to leave her bar when another man told them he had met the girl too but it was a while ago. She was a newbie and had asked him a lot of questions. He had answered the best he could. He was glad to hear she was fine.
It was the first but not the last time they heard about her. Their way never crossed hers, nonetheless, it was not unusual to arrive on a city and heard that the weirdness had been solved by some pretty young girl. Hunters talked about her too. It was so unusual for them to have a woman so young fight by their sides. It was said that, despite her young age, she had eyes who told of a rough past, bloodied like most of theirs and she knew what she was doing.
When things went to Hell, literally, they both had a though for her. When Dean died and Sam was left alone, he had thought about finding her and travelling together. But Ruby showed up and kept him busy and he forgot about the idea. After knowing exactly what she was up too, he found himself stupid. He should have search for her instead of trusting a demon.
When her brother came back from the dead, literally, they forgot the girl. The seals were broken one after the other and their minds were focused on that. They were also discovering that freaking angels were real and that concept was so hard to accept that he was kind of occulting everything else.
Then the final seal was broken, Lucifer was walking free and everything was going to Hell, even their relationship. None of them was thinking about her anymore.