Leonard, as the narrator comments at one point, is more an idea than a person. He represents everything that's thwarted by modern life – he's a romantic and relatively intelligent young man who should be leading a healthy and happy life somewhere in the English countryside, but instead, he's trapped in a dead-end job in a dead-end city, London. Leonard is basically a picture of complete frustration; he's intellectually frustrated, sexually frustrated, and economically frustrated. That's a lot of stress for one dude, and it really shows in his prematurely aged demeanor. He's an all-around sad guy, and we have to feel bad for him, even if we don't find him particularly exciting or sympathetic.
Nobody finds Leonard particularly exciting or sympathetic, really – the Schlegels adopt him because of his interesting desires to escape the quiet desperation of his life, but he doesn't really live up to his potential for interest. The thing is, he's never allowed to live up to his potential because he doesn't have the most important ingredient – money. Forster uses Leonard to show us just how destructive poverty can be; being poor is what destroys Leonard's hopes and dreams, and his intellectual ambitions. It is what crushes him so completely, and that, in turn, is what draws Helen to him (it's totally warped, we know).