Percy always keeps his promises.

He promised Ginny before she boarded the train to Hogwarts for the first time that he'd help her with her homework, if she ever asked. He would stay up late in the Common Room, helping her with Transfiguration or Potions, until both were satisfied that she understood. Even after he left home and was working at the Ministry, she would owl him with problems and he'd write her back with help or an explanation. Because Percy always keeps his promises

He promised Ron when he used to enjoy watching Percy and his father playing Wizard chess that he'd teach the young boy how to play. They would practice for hours in the attic during the warm, lazy summers Percy spent at the Burrow while he was enrolled at Hogwarts. To this day Ron's the top chess player in the entire student body, and people sometimes wonder aloud as they watch him play, how was it that he had become so good at the game? It's because Percy always keeps his promises.

He promised little George, after he came crying to him that he and his twin had been mistaken for each other yet again by their mother that he, Percy would learn to tell them apart. To this day, he's one of the only ones who can do so. And after yelling at them as Prefect during school for one time too many, he promised Fred that he'd teach them a lesson one of these days. He watched over them, if perhaps reporting them more often than necessary. He tried as hard as he could to teach them diligence and respect. Because Percy always keeps his promises.

He promised Charlie that he'd take care of his older brother's broomstick if he was allowed to ride it once a week on Sundays. Percy had no ambitions to play Quidditch, but rather liked to fly high over the cities and towns, looking down at the tiny figurine landscapes below. To this day that very broom, a Comet 100, is pristine in its antiquity, so much so that he's gotten several offers from Wizarding museums to buy it for high prices, but he will never sell it. Percy always keeps his promises.

He promised Bill, after he'd gotten his letter from Hogwarts the summer before his fifth year proclaiming him a Prefect, that he too would receive the honor. Then again in seventh year, when Bill was appointed Head Boy, he told his brother, "Bill, I'm gonna be Head Boy too someday, just like you!" And he did indeed become Prefect his fifth year and Head Boy his last. Because Percy always keeps his promises.

He promised his mother that he'd fulfill her dream of having wonderful Weasley grandchildren. He and Penelope Weasley now have three beautiful children and another on the way and make Molly happier every day. He promised his father he'd make him proud. He now works at the Ministry, and is on a pretty strong path to becoming the Minister of Magic in the relatively near future. Because Percy always keeps his promises.

He promised Penelope on their wedding day. "Will you love and honor her all the days of your life, for better or for worse, through good times and bad, in sickness and in health?" "I will." So he stayed by her side, no matter what happened. Even when Oliver Wood wrote him a letter saying that he'd loved him since fifth year but was too afraid to say so, and he was so sure Percy wasn't gay, and even if Percy could never feel the same way wouldn't he please give him a chance. Percy had never made any promises to Oliver. He owed him nothing. So he said no. Even though he was completely homosexual. Even though he'd loved Oliver since long before fifth year. Even though he'd never loved Penelope Clearwater. He stayed with her, and he always will, because Percy always keeps his promises.