Who is Scout describing in the quote "he was the most boring child I had ever met"?

Scout is referring to Francis, one of her distant relatives. He's not distant enough as far as she's concerned, given that he's such a frightful bore. Francis is Aunt Alexandra's grandson, and he has clearly been spending a lot of time round his granny because he's picked up on her disapproval of Atticus's representing Tom Robinson in court. When Atticus takes his children to Finch's Landing, Scout's not exactly thrilled to see Francis there. She's even less thrilled when Francis starts taunting her. Not only does he call her good friend Dill a "runt," for good measure he also says that Atticus is a "n****r lover" on account of his agreeing to be Tom Robinson's defense attorney.
Scout doesn't take this lying down. She curses Francis and proceeds to give him a well-deserved pasting. She's so mad at him that she has to be pulled off by members of the family. The whole episode illustrates once again just how hard it is for Scout to control her temper. It also shows us that she won't tolerate insults against the people she cares about most.