Ron's twin older brothers are the pranksters of the Weasley family. When they help Ron break Harry out of the Dursleys' house, they demonstrate a somewhat disturbing familiarity with lock picks for kids just starting their fourth year at Hogwarts. Fred leaves Ron with a lifelong fear of spiders by turning his "teddy bear into a great big filthy spider because [Ron] broke his toy broomstick" (9.148). When Ginny is clearly feeling awful for much of her first year at Hogwarts, they try to cheer her up by jumping out from behind pillars and surprising her (which mainly has the effect of frightening the living daylights out of her).
While the twins are jokey people, though, they also have hearts of gold. They are both Beaters on the Gryffindor Quidditch team. In their match against Slytherin, when a rogue Bludger targets Harry, the twins are the ones who advocate that they forfeit the match for Harry's safety. George confronts team captain Oliver Wood: "This is all your fault […] 'Catch the Snitch or die trying,' what a stupid thing to tell him —" (10.72). They do their best to stick up for Harry because they realize that his health is a lot more important than a Quidditch game. Fundamentally, the twins have the best interests of Harry and their family at heart. They're just a little awkward in showing those feelings in a non-humiliating, non-threatening manner.