Since his introduction to the show in Season 2, the character of Spike has undergone radical change, from posturing "big bad" to newly-souled defender of humanity. The heart of this change occurs during the fourth, fifth, and sixth seasons of the show, and is facilitated by the extreme marginalization that Spike experiences. Season four explores the theme of marginalization, as his being implanted with the chip separates him from not only Drusilla and Harmony, his lovers, but the rest of his kind as well. Season five focuses on the redefinition of Spike, balancing between hero and villain, ally and threat. Season six sees the new Spike pursuing the path to redemption, attempting to rejoin not the vampire world he left, but the world of Buffy and her friends. This culminates in his self-initiated quest for a soul.