Ghost was out doing Patrol on Earth, and got a Guardian loot drop. That's the origin story of everyone. You're equipment for your ghost to save the solar system from the darkness. You want to know things? Bah! When was the last time you explained to a hammer that you were using it to build a stool? If it was smart it would figure it out, and if it wasn't, oh well, it's just a hammer. Ghosts could just go find and resurrect another bullet sponge to watch it's back while it saves the solar system. Guardians, steadfast, strong, and determined, use an array of powerful weapons and traveler technology to defeat their foes. What do Ghosts use to defeat their foes? Guardians. You can always see who's in charge of a military by looking at whomever is not shooting the guns.