Yes, as strange as it might sound, I want to play a dementor.
The story would take place during the prisoner of azkaban. Your character would be in her seventh year(thus, seventeen years old). Basically, I'm thinking that my dememtor would catch the interest of your character when she sees him on the train. Perhaps she thinks them beautiful? Perhaps he stirs up a long forgotten memory that she didn't quite get to recall?
For whatever reason, she's interested enough to come to the forest that night, and he recognizes her. Deciding to indulge her for the moment, he protects her from the others and leads her away to a clearing. What happens next?
A dementor floated above the train tracks with a swarm of his kin, awaiting the arival of the train that might be harboring a dangerous human known as Sirius Black. It drew a rattling breath, drifting lower, hoping that the clouds would somehow blot out the burning sun. It may have been freezing for humans, but for his kind the heat was searing.
Soon enough the train arrived, and they circled it. He was one of the few lucky ones that got to go inside. One of the lucky ones to be near the humans and their wonderful tasting emotions. He went through the train, opening the different compartments that held the terrified creatures, whimpering at his arival. Finally, he came to a room in the back of the train, and slowly opened the door, inhaling the feelings of the person inside. The delightful sensation of feeding made his skin tingle, an all he could think of was wanting more....
Until pain flooded his being as he was forcefully knocked away by a large, shimmery animal of light, cast by one of the teachers. It lunged for him again, but he was already out of the train, joining his siblings in the air.
Hours passed, and he found himself floating around the castle, filled with humans he could not touch. He felt anger at this, knowing there was plenty of food nearby, yet unable to take it. He fumed silently, circling the surrounding forest, waiting for something, anything to happen. Perhaps they'd find the man the humans were so desperate to get a hold of, and he could go back to his home in Azakaban.