A seer is a witch or wizard with the remarkable ability to harness their inner eye, and see the future. To see this future methods such as tea leaf reading, and palm reading are used. Many can be considered a seer, but not so many are true seers. Once every century or so however, a true seer comes about, and unfortunately for Gwendolyn Cayne, she was one of the few.
Why is it unfortunate you might ask? The answers quite clear really. By being a seer, Gwendolyn never got to really lead a normal life. For example, once turning eleven, Gwen was not allowed to attend Hogwarts, because with her prophecies came memory black outs, and huge headaches, making it difficult to learn with a whole class room full of kids. It was because of this also that Gwendolyn never had a chance to make any friends, although she could be very likeable.
So for seventeen years Gwendolyn went on living a secluded life like this with just the company of her parents: Harold and Rosaline Cayne. Gwen would have probably continued living her life like this, that is, if it wasn't for the Death Eater's takeover of the Ministry of Magic. You see, under Pius Thicknesse's reign all young witches and wizards were now required to go to Hogwarts. Seeing as Gwendolyn was indeed a witch who would have gone to Hogwarts, if it had not been for the fact that she was a seer, her parents unwillingly prepared her to leave for Hogwarts.
Mean while elsewhere in the Malfoy Manner, a more sinister plot in being brewed. Sitting around a long table are the notorious death eaters, with Voldemort at the head, and Snape to his right. With one cold glare, Voldemort silenced everyone, and spoke in no more than a hiss, "I have news. This year a girl by the name of Gwendolyn Cayne will be attending Hogwarts." Before Voldemort could explain, another death eater called out, "Why do we care about her!" Sending a withering look at the death eater, Voldemort yelled, "Silence! It would do you well not to question me. As for your question, that girl is a true seer! She could possibly know what resided in the future for us. We want that information." Looking down at his feet almost sheepishly the death eater said, "I'm sorry my lord. My faith in you shall never waver again." Raising his wand Voldemort said, "We can all be sure of that. Avada Kedavra!" With a slight smirk Voldemort turned to Snape and said, "Can I trust you to watch this girl? Find out what she knows before others do?" Snape nodded and said, "You can count of me my lord." Turning his attention away from Snape, Voldemort said, "Good. You Draco shall befriend this girl. Make her trust you. Make her want to tell you thing that she'll tell no one else. And then, give the news to Snape." Draco Malfoy didn't very much like this idea, but it wasn't like he could just object, so he silently nodded. It was this very nod that sealed the doomed future of Gwendolyn Cayne.