Merry is Pippin's bestie. And, like Pippin, he's a troublemaker through and through.
But Merry's fit into the hobbit foursome is somewhere in between the all-out levity and hilarity that is Pippin, and the often serious and somber outlook of Sam. We see Merry very excited about ordering a whole pint of beer at the Prancing Pony ("This, my friend, is a pint!") and even more excited by setting off Gandalf's biggest firework. He enjoys a bit of adventure—like pilfering Farmer Maggot's crops—but he's definitely not ready for what's ahead of him.
But Merry is a bit quicker than Pippin in adjusting to the life of the Fellowship. He consoles Pippin when Strider proves that he isn't familiar with the meals known as "second breakfasts" or "elevenses."
And it's Merry who realizes that Frodo plans to leave and journey solo to Mordor. It's Merry who initiates the Pippin/Merry orc decoy, which ends in them being hauled off toward Isenguard.
So Merry is the brains behind the dynamic duo that is Merry and Pippin. He lacks the unbridled enthusiasm of Pippin, but he makes up for it with thoughtful planning. But don't think that makes him any less fun-loving—he totally lives up to his name.