Louis Weasley
When James and Fred are not around, Louis mellows out considerably. Louis plays the cello, something which James and Fred tease him for. When the topic comes up, Louis always complains how his mother is forcing him to take lessons. But I know for a fact that's not true. I was sworn to secrecy not to tell a soul when Louis begged Arthur to take him to one of those muggle concert halls to hear an orchestra play. Sweet boy. Even though he'll throw popcorn at the ghoul with James and Fred, Louis loves classical music.
I do think Louis adds a nice balance to James and Fred, though. One time, Fred got the idea to use some of those muggle, oh, what do you call them? Slate boards? Snake boards? I forget what they're called. But basically, it's a piece of wood with small wheels on the bottom, and muggles, heaven knows why, go around on them. So somehow, Fred acquired one of them and decided it would be fun to use it on the roof of the Burrow. Thank Merlin Louis stood up and made Fred stop.
Fred got grounded. In both senses of the word.