Jem and Dill are notably different in character. Dill is introduced as a rather odd and quirky individual. Scout remembers the impact that he has when he first comes to live next door and comes to join in Scout and Jem's games and imaginings.
Thus we came to know Dill as a pocket Merlin, whose head teemed with eccentric plans, strange longings, and quaint fancies. (chapter 1)
The reference to 'Merlin' suggests that Dill has almost a magical, mystical quality; this at least is how Scout remembers him, as a highly imaginative boy who brings a whole new dimension to their childhood play.
Jem, by contrast, comes across as a more solid and realistic boy, more attuned to everyday affairs and crucially, endowed with a sense of responsibility that Dill wholly lacks. Of course this stems at least partly from the fact that he is both Dill and Scout's senior, but he also appears to have a more knowing and adult attitude from the first. His sense of responsibility, as contrasted with Dill's more careless, carefree ways, is perhaps most vividly highlighted when Dill runs away and comes to the Finch house for sanctuary. Jem instantly declares that he ought to let his mother know where he is, and calls Atticus. The young Scout regards this as the ultimate act of betrayal on Jem's part, the breaching of 'the remaining code of our childhood' (chapter 14). Although at the time she resents him for it, she also instinctively realises at this point that Jem is growing up.