Dorlene
Dorcas' friend group is very overprotective, to the point where she was putting off introducing Marlene to them. They knew the two were dating because Dorcas had spent so much time talking about her, but she had yet to make a formal introduction. She knew how intimidating her friends could be, and they had driven off her girlfriends before, but she really liked Marlene and didn't want to risk that with her. But she also cared about her friends opinions a lot.
She took Marlene back to the boys dorm, and before she brought her in, she threatened her friends not to be to rude. They acted innocent, asking when they'd ever been rude to anyone, etc. Dorcas introduced Marlene and they were not at all how she was expecting. They were super friendly. Barty offered to do Marlene's herbology essay when she mentioned how she was struggling with it, despite how he never shared his homework with anyone. Pandora asked if she wanted to see the new project she was working, even though the only person who was ever allowed to see her experiments was Regulus. Evan let her beat him in chess and Regulus called her Marls and even let her call him Baby Black.
After she left, she told Dorcas how much she loved her friends. Dorcas had no idea what was going on but she was sure it was a prank of some sort. She demanded to know what they were doing, but they just said that they wanted Marlene to be comfortable because they could tell how much Dorcas liked her. Dorcas was suspicious but she had no idea what they were planning. She was on edge for weeks, determined not to let them win, but nothing ever happened.
Her friends had known how confused she would be if they acted nicely towards Marlene. They knew how much she liked her, and they did want her to be happy, but most of all, they knew that there was no use trying to chase Marlene off, as they could tell that Dorcas was already half in love, so they decided to mess with her.