That is a Red Bikini
Harry slumped down in the common room exhausted by the looming pile of homework he had to get done. Hermione barely acknowledged him as he grimaced. Lee was content reading as Harry worked on his homework. When Hermione finally finished her Transfiguration essay, she looked up at Harry.
"Who's that?" she asked, pointing at Lee.
"Lee," Harry answered, not looking up at his own Transfiguration essay, which was taking him a bit longer than it had taken Hermione. "She's Cho Chang's sister. Cho got injured in Care of Magical Creatures, so I have to watch over her for orientation."
"Ahh," was all Hermione said before beginning on her Potions essay.
The rest of the evening went fine, but Harry was beginning to feel uncomfortable in his robes. He decided to go upstairs and grab some pajamas, or something more comfortable. When he got upstairs, Ron was playing Exploding Snap with Seamus and Dean. Harry stripped down to his underwear and he opened up his trunk.
"Ron, what is this?" Harry demanded.
"That is a red bikini," Ron answered. Seamus and Dean laughed, and Ron cracked a smile.
"I know what it is," Harry snapped, shooting Seamus and Dead a sharp glare, "I just want to know why it's here instead of all my other clothes. Did someone come and take something out of here?"
"Yeah," Ron answered. "Lee came up. She said she wanted to clean out your trunk or something. She asked if it was okay if she took some of your clothes out, and I told her that you probably wouldn't mind."
"LEE!" Harry roared, heading towards the door of the boy's dormitory.
"Can't hear you!" Lee shouted back.
Harry grimaced and stepped as far out of the dormitory as he was willing to go. Sadly, that was as far as he had needed to go. As it so happened, someone had placed a skateboard right in front of the dormitory with a string attached to one of the front wheels. Before Harry could even register anything, he was in the common room wearing nothing but his underwear and a bikini in his hand. The skateboard had deposited itself neatly at Lee's feet and she quickly hid a string.
Anger filled Harry. Lee had pranked him. Lee had done this. He opened his mouth to yell at her, but he was cut off by laughter. Every boy and girl in the common room was laughing at Harry. Even Hermione had to put a hand over her mouth to resist laughing.
"Lee, give me back my clothes," Harry snarled.
"I was just trying to clean them," Lee answered apologetically. When she handed Harry the clothes, however, she shot him the death glare. "Tell anyone, and I'll make sure that Cho never even considers thinking about you again," she whispered so low that only Harry could hear.
Harry panicked when the threat reached his ears. He quickly shoved on his clothes and slunk back to Hermione. Eventually, everybody stopped staring.