Léon is sort of the best and worst possible romantic interest for Emma. They develop along parallel paths – when they meet, both are young, idealistic, and foolishly romantic, but when they are reunited in Rouen, both have been around the block once or twice. At the beginning of their actual affair, Emma has already been through her first romantic adventure with Rodolphe, and Léon has gained some experience with the prostitutes of Paris. This is both good and bad; while Léon can understand Emma’s feelings and desires, they are perhaps too similar, which is why their affair can’t last.
It’s quite possible that Rodolphe’s sole purpose in this world is to get it on with the ladies. He’s rich, handsome, lazy, super-masculine, and smooth as butter – and despite (or rather because) of all of this, Emma totally falls for his lame "sensitive guy" act. He has a specific function in the plot, which is simply to lure Emma into adultery, and thus start her on the path towards her tragic fate.