When Penny invited Percy to the party, she had a very strong feeling that he wouldn't show up. She knew of his dislike of crowds and also knew that Percy was smart enough to know that if he showed up that he could possibly encounter Fred and George. Because of the lingering possibility of knowing he wouldn't show up, Penny was slightly taken off guard to spot her best friend and ex-boyfriend standing at the entrance to the Leaky Cauldron.
The discomfort he must have been feeling was evident on his face and Penelope's grin at seeing him quickly turned into worry. Grabbing her cardigan off the back of the chair she had been sitting in, she slipped it on and walked over to where Percy stood.
"Percy, you came!" she nearly squealed with excitement and went to hug him. "I'm so glad you're here!" The worried expression had quickly been replaced with a huge smile when she actually got to him.