In addition to a few new classes, the DemonGate Companion adds a complete Life Path system to character generation. This optional component to chargen can really make your campaign shine.
A setting like DemonGate thrives when relationships, geography, and history is integrated into your character concept and the Life Path system makes this happen.
Moreover, it's not mechanically just fluff. Based on your life path, your skills, attributes, or more may be altered (usually improved!) and you may gain an NPC ally or arch enemy.
All this is really fun stuff and I highly recommend the DemonGate Companion.